This Brussels sprouts gratin has been a revelation in my kitchen: a simple weeknight upgrade that has also earned a place on our holiday table. I first developed this version one chilly November evening when I wanted the comfort of a gratin but with a vegetable-forward twist. Roasting the halved Brussels sprouts concentrates their natural sweetness and creates crispy edges, while the warm, cheesy white sauce tames any lingering bitterness and adds creamy indulgence. The result is a dish that manages to be both elegant and unapologetically cozy.

What makes this gratin special is the balance of textures and the straightforward technique. The sprouts roast in an 8"x12" casserole until slightly caramelized; that caramelization gives the gratin a nutty backbone. A quick roux-based sauce, enriched with fat-free milk, grated Parmesan, fresh thyme, and melted Gruyère, binds everything together and browns beautifully under the oven’s heat. I discovered this combination after experimenting with pantry staples and leaning into flavors my family loves — the thyme adds a woodsy lift, and the Gruyère brings a nutty, melty finish.

Ingredients

• Brussels sprouts (16 oz): Choose medium-sized heads that are firm and green with tightly closed leaves. Trim off any discolored outer leaves and slice each in half lengthwise so the cut side gets nice caramelization. Buying pre-trimmed sprouted packages is fine if they’re fresh.
• Kosher salt & black pepper: Salt seasons both the sprouts and the sauce; use kosher salt for even seasoning. Freshly cracked black pepper adds aromatic heat — adjust to taste.
• Olive oil spray: A light spray prevents sticking and helps the cut sides brown. Alternatively, toss the sprouts with 1 teaspoon of olive oil if you don’t have a spray.
• Butter (1/2 tbsp): This small amount adds flavor to the shallots and helps carry the roux. Use unsalted butter and adjust total salt to taste.
• Shallots (1/3 cup, chopped): Shallots soften and release a sweet, mild onion flavor that melts into the sauce; red onion can substitute in a pinch, but shallots are preferred.
• All-purpose flour (2 tsp): This creates the roux that thickens the milk into a clingy sauce. For a gluten-free option, use a 1:1 gluten-free flour blend.
• Fat-free milk (3/4 cup): Using fat-free milk keeps the sauce lighter while still providing body when thickened by the roux. Whole milk or 2% will yield a richer sauce if desired.
• Fresh thyme (1 tsp): Thyme’s earthy, slightly lemony note complements the sprouts beautifully. Strip leaves from the stem for the freshest flavor.
• Parmesan (1 tbsp, grated): Adds savory umami and helps the sauce cling. Use a real Parmigiano-Reggiano if possible for the best depth.
• Gruyère (2 oz, grated, divided): Gruyère melts smoothly and browns nicely; reserve half for the sauce and top with the remainder before baking for the best golden crust.

Instructions

Preheat and prepare: Preheat the oven to 400°F. Spray an 8"x12" gratin dish or small casserole with olive oil spray. Place the trimmed and halved Brussels sprouts cut side up in the dish in a single layer if possible, seasoning with 1/4 teaspoon kosher salt and a few grinds of black pepper. Lightly spray the tops with olive oil to encourage browning. This step primes the sprouts for strong caramelization during roasting. Roast the sprouts: Put the dish on the lower third of the oven and roast for 15 minutes. Remove and use tongs to toss the sprouts so different sides contact the pan, then return to the oven for an additional 10 minutes. Look for a deep golden color on the cut edges and a tender interior; total roasting time should be about 25 minutes. If your oven runs cool, add 5 minutes; if they’re browning too fast, move the dish up a rack and tent loosely with foil. Make the shallot roux: While the sprouts roast, heat a medium nonstick skillet over medium heat. Add 1/2 tablespoon butter and let it melt but not brown. Add 1/3 cup chopped shallots and cook 4–5 minutes until softened and translucent. Sprinkle 2 teaspoons all-purpose flour over the shallots and whisk constantly for 1–2 minutes to cook the flour taste and form a blond roux. The roux should smell nutty and be slightly bubbling but not darkened. Finish the sauce: Slowly whisk in 3/4 cup fat-free milk, stirring to fully incorporate the roux. Reduce heat to medium-low and simmer gently for about 4 minutes, stirring frequently, until the sauce thickens and coats the back of a spoon. Stir in 1 teaspoon fresh thyme, 1 tablespoon grated Parmesan, and half (1 oz) of the grated Gruyère until melted and smooth. Taste and adjust salt — you may need a pinch more depending on your Parmesan. Assemble and bake: Pour the warm cheesy sauce evenly over the roasted Brussels sprouts. Sprinkle the remaining 1 oz grated Gruyère across the top. Bake the assembled dish for 15 minutes at 400°F, or until the top is lightly browned and the sauce is bubbly. If you want an extra crust, place under the broiler for 1–2 minutes, watching closely so it doesn’t burn.

Storage Tips

Store leftovers in an airtight container in the refrigerator for up to 3 days. To reheat, place portions in an oven-safe dish and warm at 350°F for 10–15 minutes until hot and bubbling; this restores some crispness to the top. For freezing, cool completely, wrap tightly with plastic wrap and foil, and freeze for up to 3 months. Thaw overnight in the refrigerator and reheat as above. Avoid microwaving from frozen — it can make the texture watery — but if pressed for time, microwave briefly and then finish under the broiler for a minute to refresh the crust.

Ingredient Substitutions

If you prefer a richer sauce, swap fat-free milk for 1 cup 2% or whole milk and reduce the butter to 1/2 tablespoon; for a creamier texture, replace half the milk with 1/4 cup heavy cream. To make this vegan, use a plant-based milk (unsweetened soy or oat works best) and a vegan butter substitute, and swap Parmesan and Gruyère for high-quality vegan melting cheeses. For a nutty variation, add 1 tablespoon Dijon mustard to the sauce; it brightens and deepens the flavor without overpowering the Brussels sprouts.

Serving Suggestions

This gratin pairs beautifully with roasted pork tenderloin, herb-roasted chicken, or a simple pan-seared salmon fillet. For a vegetarian plate, serve alongside a warm farro salad tossed with toasted walnuts and dried cranberries, or a lemony quinoa pilaf. Garnish with extra fresh thyme leaves or a small sprinkle of lemon zest to brighten the richness just before serving. For holiday tables, arrange the gratin on a platter with roasted chestnuts and ribbons of prosciutto for contrast.

Cultural Background

Gratin is a French technique — a creamy, layered dish finished under high heat to form a browned crust. While traditional gratins often feature potatoes in a cream base, vegetable gratins like this one showcase a simpler, lighter approach that highlights seasonal produce. Brussels sprouts themselves have Northern European roots and gained popularity in modern North American cookery through roasting and caramelization techniques that contrast their natural bitterness. This recipe merges classic French technique with contemporary American preferences for roasted vegetables and reduced-fat dairy.

Seasonal Adaptations

In winter, boost heartiness by stirring in roasted butternut squash cubes or sliced parsnips before topping with cheese. In spring and summer, lighten the dish by adding lemon zest and substituting part of the Gruyère with fresh ricotta stirred into the sauce. Around the holidays, fold in chopped toasted chestnuts or scatter pomegranate arils over the finished gratin just before serving for a festive touch.

Meal Prep Tips

To save time, roast the Brussels sprouts and prepare the sauce a day ahead. Store them separately: sprouts refrigerated in the baking dish covered with foil, and sauce in an airtight container. When ready to bake, reheat the sauce gently on the stovetop, pour over the sprouts, sprinkle the remaining Gruyère, and bake for 15 minutes until bubbly. For single-serve meal prep, divide into four 9-oz oven-safe containers; reheat in a 350°F oven for 12 minutes.

Whether you serve this gratin as an everyday side or a centerpiece for a special meal, it’s a dependable and satisfying dish that shows how a few thoughtful techniques can turn humble ingredients into something memorable. I hope you make it, adapt it, and share it — that’s what keeps recipes alive in a kitchen.