Preheat oven to 450 F. Brown ground beef and onions together and drain. In 9 x 13 casserole dish, combine frozen peas, corn, and drained tomatoes. Add salt, pepper and thyme to taste and stir. Add browned ground beef and onions. Dissolve bullion on water and pour on top of beef and vegetable mixture. Sprinkle cheese evenly on top of ground beef and vegetable mixture (optional). Arrange tater tots in an even layer on top. Sprinkle salt, pepper, and thyme on top to taste. Bake for 30 minutes or until tots are crispy. Serves 6.