How to Make It
Preheat oven to 350°F. Prepare pasta according to package directions, cooking to al dente; drain well, and set aside.
Combine ground beef, onion, and bell pepper in a large Dutch oven over medium-high. Cook, stirring often, until beef is thoroughly cooked and onions and peppers are soft, 10 to 12 minutes. Remove from heat, and drain off any liquid. Stir in taco sauce and cooked pasta.
Toss together shredded cheeses in a medium bowl; reserve 1 1/2 cups of the cheese mixture.
Melt butter in a large saucepan over low; whisk in flour and salt until smooth. Cook, whisking constantly, 1 minute. Gradually whisk in milk; increase heat to medium, and cook, whisking constantly, until milk mixture is thickened and bubbly, 7 to 8 minutes. Stir sauce into pasta mixture. Stir in remaining 3 1/2 cups of cheese. Spoon mixture into a lightly greased (with cooking spray) 12-inch ovenproof skillet or a lightly greased (with cooking spray) 13- x 9-inch baking dish. Top evenly with reserved 1 1/2 cups cheese.
Bake in preheated oven until bubbly and golden brown, 35 to 40 minutes. Serve immediately with desired toppings.