In a large bowl, mix ground beef, grated onion, grated garlic, dried parsley flakes, salt, pepper, bread crumbs, and milk. Mix well with your hands until combined. Form into patties.

Heat a little olive oil in a pan over medium-high heat. Add patties and cook until browned on both sides and just about done.

Place the cooked patties in a slow cooker.

In the same pan you cooked the patties, add the beef gravy, ketchup, Worcestershire sauce, and pepper. Stir to combine, scraping the bottom of the pan to get all the flavorful bits.

Add the drained mushrooms to the pan and stir to combine.

Pour the gravy mixture over the patties in the slow cooker.

Cover and cook on low for 3 hours.

Serve hot with your favorite sides.