In a large skillet, cook beef and onion over medium heat until meat is no longer pink; drain. Stir in the corn, peas, ketchup, Worcestershire sauce and garlic. Reduce heat to medium-low; cover and cook for 5 minutes., Combine the cornstarch, bouillon and water until well blended; stir into beef mixture. Bring to a boil over medium heat; cook and stir until thickened, 2 minutes. Stir in sour cream and heat through (do not boil)., Spread mashed potatoes over the top; sprinkle with cheese. Cover and cook until potatoes are heated through and cheese is melted.