Brown the ground beef and onion in a large skillet over medium-high heat until the beef is no longer pink and the onion is tender. Drain any excess grease.

Stir in the garlic, salt, black pepper, and thyme. Sprinkle the flour over the beef mixture and stir until evenly coated.

Add the beef broth, tomato paste, and Worcestershire sauce. Scrape up any browned bits from the skillet and cook for 2–3 minutes, until the gravy thickens slightly.

Transfer the beef mixture to a lightly greased 6-quart slow cooker. Stir in the frozen mixed vegetables, then spread the mashed potatoes evenly over the filling.

Cover and cook on LOW for 5–6 hours, until the filling is hot and bubbling around the edges.

Sprinkle the cheddar cheese over the mashed potatoes during the last 15–20 minutes of cooking. Cover again until the cheese melts.

Let rest for about 10 minutes before serving.