nacho cheese sauce recipe

2 tablespoons unsalted butter - 2 tablespoons all-purpose flour - 1 cup whole milk - 1 cup shredded cheddar cheese - 1/2 cup shredded Monterey Jack cheese - 1/2 teaspoon garlic powder - 1/2 teaspoon onion powder - 1/4 teaspoon paprika - Salt and black pepper to taste - Optional: sliced jalapeños, diced tomatoes, or chopped green onions for garnish