In a large skillet over medium heat, cook the ground beef until it is fully browned, breaking it apart with a spoon as it cooks. Once browned, drain off any excess grease.
Lightly coat the inside of the slow cooker with nonstick cooking spray to help prevent sticking.
Add the cooked ground beef to the slow cooker along with the chopped onion, minced garlic, onion powder, frozen peas and carrots, and cream of mushroom soup. Stir everything together until the mixture is evenly combined.
Cover the slow cooker with the lid and cook on Low for about 3 to 4 hours so the flavors blend and the vegetables heat through.
During the final 2 hours of cooking, spread the prepared mashed potatoes evenly over the top of the beef mixture, covering the surface as evenly as possible.
Continue cooking until the potatoes are warmed through and the filling is hot and bubbly. Serve warm straight from the slow cooker.