Add oil to a large 10" skillet and heat over medium high heat.
If making your own patties add ground beef and onion soup mix to a bowl and mix well. Shape into 4 patties.
If using ready made store bought patties or frozen hamburger patties add the onion soup mix to the gravy mix to pour over the top.
Once oil in skillet is hot, dip both sides of patties into flour and fry patties for about 2 minutes each side until browned on the outside. Remove from heat.
In slow cooker whisk together cream of mushroom soup, milk and au jus dry mix (and onion soup mix if you didn't add to the meat).
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.