Best Broccoli Rice Casserole

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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