Cook macaroni according to package directions, adding green pepper or peas during the last 3 minutes of cooking; drain. Cover and keep warm.
Meanwhile, for sauce, melt butter over medium heat in a large skillet. Cook onion in hot butter for 4 to 5 minutes or until onion is tender. Stir in flour, salt, and black pepper. Add milk all at once. Whisk constantly until thick and bubbly. Cook, whisking constantly, for 2 more minutes. Reduce heat to low.
Add American & cheddar cheese blend and mozzarella cheese, stirring until sauce is smooth and cheese is melted. Stir the sauce and bison pepperoni into the pasta. Heat through.