au gratin potatoes recipe

4 cups thinly sliced potatoes (Yukon Gold or Russet) 1 1/2 cups shredded cheddar cheese 1 small onion, finely chopped 3 tablespoons all-purpose flour 2 cups milk 3 tablespoons unsalted butter 1 teaspoon salt 1/2 teaspoon black pepper 1/4 teaspoon garlic powder 1/4 teaspoon paprika (optional)