Preheat the oven to 375 degrees F.
Place the cauliflower into a steamer basket fitted over the pot, cover, and steam until just tender, about 5 minutes. Finely chop the steamed cauliflower.
In a small bowl, combine the bread crumbs, Parmesan, and oil.
In a large saucepan, whisk together the milk and flour until the flour is dissolved. Whisking constantly, bring the mixture to a gentle boil over medium heat. Reduce the heat to medium low and simmer until the mixture thickens slightly, 2 to 3 minutes. Stir in the cheddar, Gruyere, mustard powder, paprika, salt, black pepper, and cayenne pepper. Whisk until the cheeses are melted and the mixture is smooth, 1 to 2 minutes. Add the chopped cauliflower and macaroni and stir until well coated.
Spray an ovenproof 10-inch high-sided skillet with cooking spray. Pour the mixture into the prepared skillet. Sprinkle with the bread crumb mixture, place on a baking sheet, and bake until the top is browned and the cheese is bubbly, 35 to 40 minutes.
Homemade bread crumbs are one of those "chef's secrets" ¿ important but often overlooked detail that can make a dish. These easy-to-make crumbs are flaky, mild, and versatile¿they don't have that heavy whole-wheat taste and dense texture that most store-bought brands have. Since bread crumbs are a comfort-food cornerstone used in everything from cutlet coatings to casserole toppings, it's key to have the best.
Preheat the oven to 350 degrees F.
Place the bread in a food processor and process until fine crumbs form, 25 to 30 seconds. Place the crumbs on a baking sheet, spreading them evenly. Bake until golden brown, about 12 minutes. Store the bread crumbs in an airtight container for up to 2 weeks.
Reprinted with permission from Comfort Food Fix by Ellie Krieger, copyright (c) 2011. Published by Wiley.