Preheat the oven to 400 degrees F. Place the cauliflower florets on a large baking sheet, toss with the olive oil, and sprinkle with Italian seasoning and garlic powder. Roast in the preheated oven for 20 minutes, turning after 10 minutes.
Meanwhile, start the water for the macaroni in a large pot, and prepare according to package direction. While the water is coming to a boil, begin to prepare the cheese sauce.
Melt the butter in a large saucepan over low heat. Stir in the flour and cook for one minute. Slowly whisk in the milk, whisking constantly, until the milk is steaming. Slowly add the shredded cheeses, a small handful at a time, whisking constantly, until all the cheese is melted and the sauce is bubbly.
Add the roasted cauliflower to the cooked pasta, pour in the cheese sauce and stir. Let sit for 5 minutes to absorb the sauce. Season with black pepper and garnish with parsley, if desired.