01 - Preheat oven to 400°F. Lightly grease a medium baking dish.
02 - Bring a large pot of salted water to a boil. Add cauliflower florets and cook for 3 minutes. Add elbow macaroni and continue boiling together for an additional 6 to 7 minutes, until pasta is al dente and cauliflower is just tender. Drain well and set aside.
03 - In a medium saucepan over medium heat, melt butter. Whisk in flour and cook, stirring constantly, for 1 minute. Gradually pour in milk, whisking steadily to avoid lumps, and cook until the mixture thickens, about 3 to 5 minutes.
04 - Remove sauce from heat. Stir in sharp cheddar, Parmesan, mozzarella cheeses, Dijon mustard, garlic powder, nutmeg, salt, and black pepper until the cheeses are melted and the sauce is smooth.
05 - Mix the drained pasta and cauliflower with the cheese sauce, ensuring even coating. Pour the mixture into the prepared baking dish.
06 - In a small bowl, toss breadcrumbs with olive oil until coated. Evenly sprinkle breadcrumbs over the assembled casserole.
07 - Bake uncovered for 15 minutes, until bubbling and golden brown on top. For extra crunch, broil for 1 to 2 minutes if desired.
08 - Allow casserole to cool for 5 minutes before serving. If using, garnish with chopped parsley.