In a large bowl, combine beef, 1/4 can mushroom soup, bread crumbs, egg, onions, and steak seasoning. Mix thoroughly and shape into 4 oval patties.
Heat oil and 1 tablespoon butter in large skillet over medium-high heat. Brown patties on both sides and transfer to a plate.
Add remaining butter and cognac (remove pan from heat when adding cognac.) Saute mushrooms for 7 to 8 minutes. Add beef stock and whisk in gravy mix until smooth. Stir in remaining mushroom soup.
Return patties to skillet and spoon gravy over top. Cover pan and simmer for 20 to 25 minutes.
Serve Salisbury Steaks and Mushroom Gravy over hot cooked rice.