Brown ground beef in a large sauté pan over medium-high heat. Drain; return ground beef to pan. Stir in frozen vegetables, 1/4 cup water and onion. Reduce heat to medium, cover and simmer, stirring occasionally, 5 minutes or until vegetables are thawed.
Meanwhile, prepare mashed potatoes according to package directions, using 1-1/2 cups water, salt, butter and milk. Cover and set aside.
Stir soup, Worcestershire sauce and thyme into meat and vegetables. Spread prepared mashed potatoes on top. Sprinkle with cheese. Reduce heat to low, cover and heat through, about 5 minutes.