Preheat the oven to 350˚F degrees. In a large skillet, melt the butter over medium heat. Add the onion and garlic and cook, stirring with a wooden spoon, until softened, 3 to 4 minutes. Sprinkle over the flour, dry mustard, and cayenne and stir to mix it in well. Continue to cook for 1 minute.

Next, add the milk, stirring constantly, and cook until thickened, about 2 minutes. Add the cream cheese and parmesan, stirring until totally combined. Stir in the pepper, salt, and paprika.

Add the processed cheese, stirring until completely melted. Next, add 1 ½ cups of the cheddar and stir until melted. Then, fold in the broccoli and pimentos.

In a large baking dish, create a base with half of the rice. Top with half of the broccoli cheese sauce. Repeat with the remaining rice, then the remaining sauce. Sprinkle the remaining 1 ½ cups cheddar evenly over the top of the casserole. Bake until bubbly, about 30 minutes.