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Preheat Oven: Arrange oven rack to center position. Preheat oven to 400 degrees F. Spray a 2 quart gratin or baking dish with non-stick cooking spray. Set aside.
Prepare the topping: In a small mixing bowl, combine the panko, pine nuts, 1/2 cup gruyere, and 1/4 cup parmesan. Season with 1/8 teaspoon of salt and a pinch of pepper. Add in the melted and cooled butter. Stir well to combine. Set aside.
Par-bake Vegetables: Line a large, rimmed baking sheet with aluminum foil for easy clean up. Add brussels sprouts and onions. Drizzle with oil and season with 1/2 teaspoon salt and 1/4 teaspoon of pepper. Toss well to coat. Spread the vegetables out in an even layer. Bake for 15-20 minutes, tossing halfway through cooking, or until vegetables are almost fork tender. Remove from oven and transfer to prepared baking dish.
While the vegetables are baking, Make the Sauce (SEE NOTES): In a medium-size saucepan, melt the butter over medium heat. Add the flour and stir continuously for 1-2 minutes or until the mixture is blonde in color. While whisking constantly, gradually add in the cream. Add the nutmeg and season with 1/4 teaspoon of salt and 1/8 teaspoon of pepper. Bring mixture to a gentle simmer and reduce heat as needed to maintain a gentle simmer. Cook, whisking frequently, until the sauce is thickened and coats the back of a spoon, about 5-8 minutes. Turn off the heat. Add in 3/4 cup of gruyere cheese. Whisk until smooth. Add in 1/4 cup parmesan. Whisk until the cheeses are fully incorporated. Taste and adjust for seasoning with salt and pepper.
Assemble the gratin: Pour the sauce evenly over the brussels sprouts and onions in the prepared baking dish. Use a spatula to spread the sauce out in an even layer. Sprinkle the crunchy topping evenly over the sauce.
Bake: Bake gratin UNCOVERED, for 20-25 minutes or until the sauce is bubbling and the top is golden brown. If the top starts to brown too quickly towards the end of baking, lightly tent the dish with aluminum foil.
Rest and Serve: Allow to rest for 5 minutes before serving. Garnish with more parmesan and fresh thyme! Enjoy!