Brussels Sprouts Gratin with Quinoa and Caramelized Onions

Caramelized onions lend a deep, savory note to this Brussels sprouts gratin. They are folded into a rich sauce of heavy cream, Dijon mustard, and vegetable broth, then baked beneath a crisp browned-butter breadcrumb topping.

Quinoa adds protein and texture, making this casserole a satisfying vegetarian main or a hearty side dish.

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If you want this recipe to turn out perfectly the first time, use the sections below that help you most — ingredients, equipment, tips, and the recipe card at the end.

I’ve always loved a gratin: warm, unpretentious, and comforting. Some gratins can stand alone as a filling main (for example, a Swiss chard and quinoa gratin). Others act as classic side dishes on holiday tables, like creamy potato au gratin or a celery root and potato version. Gratins work with a wide range of vegetables through the seasons — roots in winter, squash in autumn, and tomatoes in summer.

🤓 What is a gratin?

A gratin is a baked dish finished with a crusty top, usually made from breadcrumbs, cheese, or both. The defining feature is a browned, gratin-style topping that creates a contrast in texture with the soft, cooked vegetables beneath. Gratins are typically baked in a shallow pan and often given a final blast under the broiler to develop a crisp, golden crust.

Disclosure: this post may reference products I use in the kitchen. I may earn a small commission if you purchase through those references, at no extra cost to you.

⏰ Tips to Simplify and Save Time

• Make the quinoa ahead and store it in an airtight container in the refrigerator for up to five days.
• Caramelized onions keep well in the fridge for about a week and can be made in advance.
• Some supermarkets sell cooked frozen quinoa that only needs reheating; this saves a step.
• Use a food processor with a slicing disc to shave Brussels sprouts quickly, or buy pre-shredded sprouts from the produce section.
• If using frozen sliced Brussels sprouts, thaw and drain them before combining with the other ingredients for best texture.

Ways to Use Gratins

• Gratins are ideal for repurposing leftover vegetables such as cauliflower, broccoli, cooked greens, or potatoes.
• They work particularly well with vegetables that have a naturally bitter edge — Brussels sprouts, kale, Swiss chard, mustard greens, or broccoli rabe — where a creamy sauce and crunchy topping balance flavors.
• Gratins are easy to prepare ahead for casual dinner parties; rewarm them gently when guests arrive.

More Brussels Sprouts Recipes

If you like this gratin, you might also enjoy pan-roasted Brussels sprouts with balsamic-glazed onions and poached eggs for a simple, flavorful main.

Brussels Sprouts Gratin with Quinoa

One-dish vegetarian gratin of shaved Brussels sprouts and quinoa — hearty enough for a vegetarian main or comforting as a side at a winter meal.

Prep Time: 20 mins

Total Time: 1 hr 20 mins

Course: Vegetable Main or Side

Cuisine: American, French

Servings: 8

Calories: 349.2 kcal

Author: Susan Pridmore

Equipment

• 9″ x 11″ casserole dish (or similar)
• Chef’s knife
• Medium skillet
• Tongs
• Food processor (optional for slicing Brussels sprouts)
• Large bowl
• Small skillet (for browned-butter breadcrumbs)

Ingredients

Brussels Sprouts

• 1 large yellow onion, thinly sliced
• 4 tablespoons unsalted butter, divided
• 1 cup cooked quinoa
• 2 pounds Brussels sprouts, hard stem bottoms trimmed
• 1 cup dried cranberries
• 1 cup vegetable broth
• 1/2 cup heavy whipping cream
• 1 tablespoon Dijon mustard
• 1 teaspoon celery salt
• 1/4 teaspoon ground black pepper
• 1/4 teaspoon kosher salt
• 1 1/2 cups browned butter breadcrumbs

Instructions

1. Preheat: Preheat the oven to 350º F and butter a casserole dish.
2. Caramelize the onions: Cut the onion in half and thinly slice. Melt 2 tablespoons of butter in a medium-large skillet over medium heat and add the onion slices. After a few minutes reduce the heat to medium-low. Let the onions sit and gently brown; toss occasionally to encourage even browning. Cook slowly until they are a deep caramel color — this can take up to an hour if you cook very gently. If you prefer a quicker method, keep the heat at medium and watch carefully to avoid burning.
3. Cook quinoa (if needed): If you do not have cooked quinoa, rinse the seeds and simmer until tender, then let them cool slightly before using.
4. Prepare the Brussels sprouts: Thinly slice the Brussels sprouts on a food-processor slicing disc or slice them thicker with a sharp knife. Place the sliced sprouts in a large bowl and add the dried cranberries, caramelized onions, and cooked quinoa.
5. Make the sauce: In the same skillet used for the onions, melt the remaining 2 tablespoons butter. Add the vegetable broth, heavy cream, Dijon mustard, celery salt, kosher salt, and ground black pepper. Whisk and heat just until the mixture begins to simmer. Pour the hot sauce over the Brussels sprouts and quinoa mixture, tossing to coat thoroughly. Transfer the mixture to the buttered casserole and spread into an even layer.
6. Bake: Bake until the Brussels sprouts are tender, about 45 minutes. If you prefer a crispier top, broil briefly at the end to brown the crumbs.
7. Topping and serving: Sprinkle the browned-butter breadcrumbs over the baked gratin for a crunchy finish. Alternatively, toss panko breadcrumbs with melted butter, salt, and pepper and spread them on top before baking. Serve warm.

Nutrition

Calories: 349.2 kcal | Carbohydrates: 47 g | Protein: 13.9 g | Fat: 13.3 g

Saturated Fat: 7.5 g | Polyunsaturated Fat: 1.4 g | Monounsaturated Fat: 3.3 g | Trans Fat: 0.2 g

Cholesterol: 31.9 mg | Sodium: 686.7 mg | Potassium: 892.2 mg

Fiber: 12.3 g | Sugar: 5.5 g | Vitamin A: 1314.9 IU | Vitamin C: 97.9 mg

Calcium: 137.1 mg | Iron: 4.2 mg