In a large bowl, combine ground beef, breadcrumbs, whole milk, egg, Worcestershire sauce, salt, black pepper, and garlic powder. Mix gently until just combined.
Form the mixture into 8 equal-sized patties and set aside.
Heat olive oil in a large skillet over medium heat. Add the beef patties and cook for 2-3 minutes on each side until browned but not cooked through. Remove patties and set aside.
In the same skillet, add sliced onion and mushrooms. Sauté for 5 minutes until softened. Add minced garlic and cook for 1 more minute.
Transfer the sautéed onions, mushrooms, and garlic to the crock pot. Place the browned beef patties on top.
Pour beef broth over the patties and vegetables.
Cover and cook on low for 6 hours.
About 30 minutes before serving, mix flour and water in a small bowl until smooth. Stir the mixture into the crock pot to thicken the gravy.
Cover and continue cooking for the remaining 30 minutes.
Serve the beef patties with the onion and mushroom gravy spooned over the top.