Step 2

Preheat the oven to 400°F. Grease a 9-by-13-inch casserole dish with 1 tablespoon of the butter.

Step 3

In a medium pot, heat 1 tablespoon of the butter over medium heat. Add the onion and sauté until translucent, 4 to 5 minutes. Add the garlic and cook until fragrant, 1 minute more.

Step 4

Add the remaining 3 tablespoons butter to the pot and melt completely. Stir in the flour to combine. Cook, stirring occasionally, until the mixture is lightly golden in color, 2 to 3 minutes.

Step 5

Add the milk gradually to the pot, using a whisk to combine. Do the same with the cream and then bring the mixture to a simmer, whisking constantly until the sauce is thick enough to coat a spoon, 5 to 7 minutes.

Step 6

Season the sauce with the nutmeg, salt and pepper. Add the broccoli and cauliflower to the pot and toss well to fully coat.

Step 7

Pour the vegetables-and-sauce mixture into the prepared casserole dish. Sprinkle both grated cheeses evenly over the surface.

Step 8

Bake until the sauce is bubbly and the cheese is golden, 35 to 40 minutes. Let sit 5 to 10 minutes before serving warm.