Preheat oven to 400°F. In a large skillet, cook ground beef and diced onion over medium-high heat until beef is browned and onion is softened, about 8 minutes. Drain excess fat.
Add minced garlic and cook for 1 minute until fragrant. Sprinkle flour over the meat mixture and stir well to combine, cooking for 2 minutes.
Pour in beef broth and Worcestershire sauce, stirring constantly until mixture thickens, about 3-4 minutes. Add frozen mixed vegetables, thyme, salt, and pepper, then stir to combine.
Transfer the beef mixture to a 9x13-inch baking dish, spreading it evenly across the bottom.
Arrange tater tots in a single layer over the beef mixture, covering it completely.
Bake uncovered for 25-30 minutes until tater tots are golden brown and crispy. Sprinkle shredded cheddar cheese over the top and return to oven for 5 minutes until cheese is melted. Let stand 5 minutes before serving.