- 5 tablespoons unsalted butter, divided
- 1 sweet onion, finely chopped
- 2 cloves garlic, minced
- 1/4 cup all-purpose flour
- 1 1/2 cups whole milk
- 1/2 cup heavy cream
- 2 sprig rosemary
- 1/4 teaspoon nutmeg
- Kosher salt
- Freshly ground black pepper
- 1 head broccoli, cut into 1-inch pieces (about 12 ounces total)
- 1 head cauliflower, cut 1-inch pieces (about 12 ounces total)
- 1 cup Gruyère or Swiss cheese, grated
- 3/4 cup grated Parmesan cheese, grated
- Preheat oven to 400 degrees.
- Grease a large casserole dish with 1 tablespoon of butter.
- In a medium pot, heat 1 tablespoon butter over medium heat. Add onion and cook until translucent, 4-5 minutes. Add garlic and cook until fragrant, 1 minute more.
- Add remaining 3 tablespoons of butter to the pot and melt. Stir in the flour to combine. Cook, stirring occasionally, until the mixture is lightly golden in color, 2-3 minutes.
- Add the milk gradually to the pot, using a whisk to combine. Do same with the cream, then bring mixture to a simmer, whisking constantly until the sauce is thick enough to coat a spoon, 5- 7 minutes. Season the sauce with nutmeg, salt, and pepper.
- Add broccoli and cauliflower to the pot and toss well to fully coat. Then, add broccoli cauliflower sauce mixture into the prepared casserole dish. Sprinkle both grated cheeses evenly over the surface.
- Bake until the sauce is bubbly and the cheese is golden, 35-40 minutes. Let rest 10 minutes before serving. Enjoy!