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["Preheat the oven to 400u00b0 F.", "Line a large baking sheet with a silicone baking mat or parchment paper. Set aside.", "Spray a 9x13"" baking dish with cooking spray. Set aside.", "Using a cheese grater or food processor fitted with a grating attachment, grate the cheddar and Gruyere cheeses.", "Place the grated cheese in a large bowl, and add the Parmesan cheese. Set aside.", "Chop the cauliflower into 1"" florets.", "Arrange the florets onto the lined baking sheet.", "Drizzle with olive oil and season with salt and pepper. Stir with a wooden spoon or rubber spatula to distribute the oil.", "Roast the cauliflower for 20 minutes, until it is able to be pierced with a fork but not tender. Remove from the oven and set aside.", "Reduce the oven temperature to 350u00b0 F.", "In a large pot over medium heat, melt the butter.", "Once melted, add the flour. Whisk to thoroughly combine.", "Continue whisking for 1-2 minutes, until the roux turns slightly golden and begins to smell toasty.", "Slowly stream the milk into the pot while continuing to whisk.", "Cook the mixture, whisking steadily for 4-5 minutes, until it begins to thicken and bubble.", "Once bubbling, turn off the heat. Add the ground mustard, garlic powder, salt, and black pepper to taste.", "Add 3/4 of the cheese to the pot one handful at a time, whisking slowly after each addition. Reserve a generous 1 cup for topping.", "Once the sauce is smooth, add the roasted cauliflower to the pot. Stir until combined.", "Pour the contents of the pot into the greased baking dish.", "Sprinkle the reserved cheese in an even layer over the top of the cauliflower. Transfer the dish to the oven.", "Bake for 15-18 minutes, until the edges are bubbling and the top is lightly browned.", "Remove the dish from the oven. Allow it to rest for 15 minutes before serving."]
