Preheat oven to 375 degrees F. Grease a 2-quart rectangular baking dish or four 16-ounce individual casseroles; set aside. In a large saucepan, combine beef tips with gravy, frozen vegetables, soup, Worcestershire sauce, onion, thyme, and pepper. Bring to boiling over medium heat, stirring occasionally. Transfer mixture to the prepared baking dish.
Stir mashed potatoes to soften. Spoon potatoes into eight mounds on top of meat mixture. Sprinkle with cheese.
Bake, uncovered, for 20 to 25 minutes or until heated through. Let stand for 10 minutes before serving. Makes 4 servings.