Preheat the oven to 425°F and spray a 9x13-inch casserole dish with nonstick cooking spray.
Prepare the cauliflower. In a large saucepan, add 1 ½-2 inches of water over medium-high heat. Once hot, place the chopped cauliflower into a steamer basket and add it to the pan. Cover with a lid and cook the cauliflower for 5 minutes. Lift the steamer from the pan and drain well. Set aside.
1 head of cauliflower
In a separate saucepan, make a roux. Melt the butter, add the garlic, and cook for 1 minute. Whisk in the flour to make a paste, followed by the milk. Cook until it begins to thicken.
2 tablespoons unsalted butter, 1 tablespoon minced garlic, 3 tablespoons all-purpose flour, 2 cups whole milk
To the roux, add the cream cheese, stirring to combine as it melts. Turn off the stove. Stir in the salt and pepper and add 1 cup each of cheddar and Monterey jack cheese. Stir until the cheese melts.
4 ounces cream cheese, 1½ cups shredded cheddar cheese, ½ teaspoon kosher salt, ½ teaspoon ground black pepper, 1½ cups shredded Monterey jack cheese
Place the steamed cauliflower in the prepared casserole dish and pour the cheese sauce over the cauliflower.
Add dollops of sour cream over the cauliflower.
⅔ cup sour cream
Sprinkle the top of the casserole with the cooked bacon and remaining cheese.
5 slices bacon
Place in the oven and bake until bubbly and lightly browned, about 10-15 minutes.
Remove and garnish with chives. Serve immediately.
1 tablespoon diced chives