Best Broccoli Rice Casserole

Name: Best Broccoli Rice Casserole
Ingredients: 4 tablespoons (1/2 stick) salted butter; 1 medium yellow onion, finely diced; 1 clove garlic, grated; 4 tablespoons all-purpose flour; 1/2 teaspoon dry mustard; 1/4 teaspoon cayenne; 3 cups whole milk; 4 ounces cream cheese, at room temperature; 1/2 cup grated Parmesan; 1 teaspoon freshly ground black pepper; 1/2 teaspoon kosher salt; 1/2 teaspoon paprika; 8 ounces processed cheese, cubed; 3 cups grated sharp Cheddar; 8 cups small broccoli florets; 6 ounces diced pimentos, drained; 2 1/2 cups cooked long-grain rice