Add oil to a large 10" skillet and heat over medium high heat. Add ground beef and onion soup mix to a bowl and mix well. Shape into 4 patties and dredge each side into flour, lightly coating each patty.
Once oil in skillet is hot, dry patties for about 2 minutes each side until browned on the outside. Remove from heat. Add cream of mushroom soup, milk and au jus dry mix to a slow cooker and mix well.
Add ground beef patties and spoon mushroom soup mixture over the top so they are coated. Cover and heat on low for 4-6 hours.
When cooking is complete, serve steak an sauce over a bed of mashed potatoes and top with chopped fresh parsley if using.