Brown meat in large skillet; drain. Return meat to skillet.
Add milk, ketchup, mustard and onion powder; mix well. Bring to boil. Stir in Shell Pasta; return to boil. Cover; simmer on medium-low heat 10 min. or until pasta is tender.
Stir in Cheese Sauce and sour cream until blended. Add tomatoes and onions; mix well.