1. WHISK FLOUR INTO BUTTER
Preheat oven to 400°. Microwave milk at HIGH for 1 1/2 minutes. Melt butter in a large skillet or Dutch oven over medium-low heat; whisk in flour until smooth. Cook, whisking constantly, 1 minute.
2. WHISK IN WARM MILK
Gradually whisk in warm milk, and cook, whisking constantly, 5 minutes or until thickened.
3. WHISK IN CHEESE
Whisk in salt, black pepper, 1 cup shredded cheese, and, if desired, red pepper until smooth; stir in pasta. Spoon pasta mixture into a lightly greased 2-qt. baking dish; top with remaining cheese. Bake at 400° for 20 minutes or until golden and bubbly.