Cauliflower gratin isn’t exactly reinventing the wheel when it comes to casseroles, but this version certainly does, with an all-around perfect Gruyère cheese sauce and alternative baking times for the most perfect results.
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Preheat the oven to 400 degrees.
Toss the cauliflower with the 4 tablespoons of melted butter then place it in a large roasting pan that has been greased with cooking spray. Season liberally with salt and roast in the oven for 30 minutes or until soft and slightly browned. Remove the pan from the oven, and set aside.
Reduce the oven temperature to 350 degrees.
In a medium saucepan set over medium heat, melt 2 tablespoons of the butter. Add the onion and sauté until tender, about 5 minutes. Add the garlic and sauté for 1 minute. Add the flour and cook, whisking constantly, for 1 minute. Gradually add the milk while whisking constantly. Bring the mixture to a boil. Reduce the heat to low, and simmer stirring frequently, until the mixture has thickened, about 10 minutes. Remove the pan from heat, and add 2 cups of the Gruyère and the parsley. Stir until the cheese is melted and the sauce is smooth. Season with salt and pepper. Add the roasted cauliflower and stir to coat. Pour the mixture into a 9-by-13-inch baking dish.
Melt the remaining 2 tablespoons of the butter. In a small mixing bowl, combine the butter and breadcrumbs. Add the remaining 1 ½ cups of cheese and the chives, and stir to combine. Sprinkle the topping evenly over the gratin.