In a large resealable plastic bag, place flour, seasoned salt, black pepper, garlic granules and onion powder. Stir the flour and seasonings together and add the beef. Seal the bag and shake several times to coat the beef with seasoned flour. Brush off excess flour.
Heat half of the rapeseed oil in a large frying pan over medium-high heat. Fry half of the beef, turning them over until all sides are nicely browned, 5 to10 minutes. Remove the browned beef to a plate and repeat with the rest of the rapeseed oil and beef. Transfer to plate and cook and stir the onion in the hot frying pan until translucent, about 5 minutes.
Whisk together beef stock, mushroom soup and skimmed milk until smooth. Return the beef to the frying pan together with any juices on the plate, add the mushrooms and pour the stock mixture over the beef and mushrooms. Turn heat to low and simmer until the meat is very tender, about 3 hours. For a thicker gravy, mix cornflour and water to make a smooth paste and stir into the gravy. Simmer about 2 minutes to thicken.