Step 1

Preheat the oven to 425°F.

Step 2

Place brussel sprouts on an 8 x 11 rimmed baking dish, drizzle with olive oil and season with salt, pepper and garlic powder. Toss to coat evenly, and then roast until golden brown, about 20-25 minutes, tossing halfway through.

Step 3

In a small saucepan over medium heat, melt the butter. Add the onions and cook until translucent, about 3-5 minutes. Add the flour and mustard powder and stir until the flour dissolves, about 1-2 minutes. Slowly pour the milk and stir with a wooden spoon until the sauce thickens. Add ¼ cup gruyere cheese and stir until well blended.

Step 4

When the brussel sprouts are done roasting, pour the sauce over the brussel sprouts. Sprinkle remaining gruyere cheese on top.

Step 5

Return the brussel sprouts to the oven and cook until the cheese is melted and bubbly, about 15 more minutes. Serve warm.