Preheat oven to 350°. Cook pasta according to package directions.
Meanwhile, in a large skillet over medium heat, cook chicken, onion, salt and pepper in oil until chicken is no longer pink. Stir in Alfredo sauce; bring to a boil. Stir in cheese, salsa, milk and taco seasoning.
Drain pasta; toss with chicken mixture. Divide between 2 greased 8-in. square baking dishes. Cover and bake until bubbly, 30-35 minutes. Freeze option: Cover and freeze unbaked casserole up to 3 months. To use, thaw in refrigerator overnight. Remove from refrigerator 30 minutes before baking. Preheat oven to 350°. Bake casserole, covered, until bubbly, 50-60 minutes.