In a bowl, mix the beef, egg, breadcrumbs, garlic powder, onion soup mix, and onions. Mix together until incorporated.
Measure the mixture in ½ C amounts, and form into patties. Fry in a pan until browned on both sides.
Carefully stack in the slow cooker. In a container, mix the gravy mix, cream of mushroom soup, water and mushrooms. Pour on top of the patties and cover.
Cook on low for 5 – 6 hours.
After cook time is complete, remove the patties from the cooker, and make a slurry of the cornstarch with 1 tablespoon of water. Pour into the cooker and mix well.
Return patties to the mixture and cook for an extra ½ hour for the sauce to thicken.