In a large bowl, gently use your hands to mix together ground beef, onion soup mix, breadcrumbs, and milk.
Shape meat into 4 patties.
Heat about 1 tablespoon of oil in a large skillet over medium-high heat. Cook patties for about 3 minutes per side, or just until browned (but not cooked through).
Place onions and mushrooms in the bottom of a large slow cooker. Place patties on top.
In a medium bowl, whisk together condensed soup, dry au jus mix, and water. Pour sauce into the slow cooker.
Cook on LOW for 3-4 hours.
