Place the sliced onions and mushrooms in the bottom of the slow cooker
Place all the ingredients for the beef patties in a medium-sized bowl and mix them using a fork or your hands, until all ingredients are well combined. Do not overwork the meat. Divide it evenly into 4 patties.
Heat the olive oil in a large skillet over medium-high heat and brown the patties on both sides for about 3 mins on each side. Place them into the crockpot on top of the mushrooms and onions.
In a bowl or large measuring cup, whisk together the beef broth, ketchup, Worcestershire sauce, Dijon mustard, fresh parsley, and dry brown gravy mix. Pour this mixture into the crockpot, place the lid on and cook on low for 5 hours, or on high for 3.
In a small bowl, dissolve the cornstarch into the water. Remove the patties from the crockpot and add the cornstarch slurry to the pot. Set it on high and cook the gravy until thickened. Return the steaks to the pot and cover them with the gravy. Serve with fresh parsley on top.