In a large bowl, whisk together the French onion soup, cream of mushroom soup, Worcestershire sauce, garlic powder, and black pepper.
Spray the crockpot with nonstick cooking spray.
Place the frozen meatballs and sliced onions in the slow cooker. Stir to coat everything in the sauce.
Cover and cook on LOW for 6-7 hours or HIGH for 2-3 hours, until the meatballs are hot and tender.
If you want a thicker sauce, mix the cornstarch with water and stir it into the crockpot during the last 30 minutes of cooking.
Sprinkle shredded Gruyère or Swiss cheese over the meatballs, cover, and let it melt for about 10-15 minutes.
Garnish with fresh parsley if desired. Serve over mashed potatoes, egg noodles, or with crusty French bread.
Notes
How to store leftovers
Refrigerate - Got leftovers? Store them in the fridge for up to three days in an airtight container.
Freezer - Freeze the leftover meatballs in a freezer safe bag or container for up to three months. For best results, thaw them in the fridge overnight before reheating.
Reheat - Simply warm up the meatballs on the stove top or in the microwave. I love meals that reheat in the microwave! So, easy!