Add the sliced onions, mushrooms, and garlic to the bottom of the slow cooker.

In a large bowl, combine the ingredients for the beef patties (leave out the oil), then form the mixture into 8 patties.

Heat a large skillet over medium-high heat and brown the patties on both sides, about 3–4 minutes per side. They don’t need to be fully cooked, just nicely browned. Transfer them to the slow cooker.

In a bowl, whisk together the beef broth, cream of mushroom soup, and beefy onion soup mix until smooth. Pour the mixture over the patties.

Cover and cook on LOW for 4–5 hours, until the beef is cooked through and tender.

Spoon the rich gravy over the patties before serving.