In a large bowl, use your hands or a fork to gently combine the ground beef, onion soup mix (see note), breadcrumbs, and milk. NOTE: use just half of the packet of onion soup mix for a less salty dish.
Shape the meat into 4 patties.
Heat the olive oil in a large skillet over medium-high heat. Cook the patties for about 3 minutes per side, or just until browned (but not cooked through).
Place the onions and mushrooms in the bottom of a large slow cooker. Arrange the browned patties on top.
In a medium bowl, whisk together the condensed soup, dry au jus mix (see note), and water. NOTE: Use just half of the packet of au jus mix for a less-salty dish.
Pour the gravy into the slow cooker.
Cover the slow cooker and cook on LOW for 3-4 hours. Give the gravy a stir and spoon over the patties. Garnish with chopped fresh parsley, if desired.
Serve the patties and mushroom gravy over mashed potatoes or noodles.