In a large bowl combine ground beef, egg, breadcrumbs, milk, garlic powder, onion powder, salt, and pepper. Mix thoroughly with hands and shape into 6 patties.
Lightly spray a large skillet with nonstick cooking spray and add patties in a single layer. Cook over medium-high heat for 3 minutes on each side until lightly browned.
Place mushrooms and sliced onions in the bottom of the slow cooker and add browned patties on top.
Pour gravy and beef broth into slow cooker and sprinkle Lipton onion soup mix over patties. Cover and cook on low for 4-5 hours.
Remove patties from the slow cooker and cover to keep warm.
In a small cup whisk cornstarch and cold water until smooth and pour into slow cooker.
Turn slow cooker to high, and cook for 5 minutes, stirring occasionally until sauce thickens.
Serve patties over mashed potatoes and spoons thickened gravy over patties. Enjoy!