3 tablespoons butter, divided
1 cauliflower (about 1 1/2 pounds), separated into small florets
Salt and black pepper
1 cup milk
1/3 cup heavy cream
2 tablespoons flour
1/2 cup grated Gruyere cheese or swiss (about 2 ounces)
Heat oven to 375 degrees.
Melt 1 tablespoon butter in a baking sheet in the oven. When it melts, add cauliflower, season with salt and pepper,and toss to combine. Roast, stirring occasionally, until cauliflower is mostly tender and lightly browned, 25 – 30 minutes.
Warm milk and cream in a microwave safe bowl, about 2 minutes. Melt remaining butter in a medium saucepan over medium-low heat. Add flour and cook, whisking constantly, until mixture turns golden brown, 3 – 4 minutes. Gradually whisk warm milk and cream into butter mixture and continue to cook, whisking frequently, until sauce has thickened slightly, about 5 minutes. Remove from heat and add half of the cheese.
Transfer cauliflower to an 8 or 9 inch square baking dish and pour the béchamel sauce evenly over. Sprinkle the remaining cheese on top. Bake until the sauce is bubbling and the cheese is golden brown, about 25 minutes.