Brown the ground lamb or beef with the onion and carrots in a skillet over medium heat until the meat is cooked through. Stir in the garlic. Drain any excess fat.
Transfer the cooked meat to the Crock Pot. Add frozen peas, corn, diced tomatoes, beef broth, tomato paste, Worcestershire sauce, dried thyme, salt, and pepper. Mix well.
Cover with the lid and cook on low heat for 4-6 hours, allowing the flavors to meld and the vegetables to tenderize.
Once the filling is ready, spread the mashed potatoes evenly over the top.
Cover and continue cooking for an additional 30 minutes to 1 hour, or until the mashed potatoes develop a golden crust.
Serve warm and enjoy this delightful slow-cooked Shepherd's Pie straight from the Crock Pot.