In a large bowl, mix together the ground beef, onion soup mix, bread crumbs, and milk using your hands. Shape into 8 patties.
Heat a large skillet over medium-high heat, coated in cooking spray. Quickly brown patties on both sides in the hot skillet.
Coat the sides and bottom of slow cooker with cooking spray.Place browned patties into the slow cooker stacking alternately like a pyramid.
Using pattie skillet, saute onions until slightly caramelized. Add mushrooms and saute for about 2 minutes. Add cream of chicken soup, au jus mix, and broth/consomme to skillet; de-glaze pan. Pour over the meat.
Cook on the Low setting for 4 or 5 hours, until ground beef is well done.