In a large resealable plastic zipper bag, place flour, seasoned salt, black pepper, garlic powder, and onion powder. Stir the flour and seasonings together, and add the beef stew meat. Seal the bag and shake several times to coat the beef cubes with seasoned flour. Brush off excess flour.
Heat half of the canola oil in a large skillet over medium-high heat. Fry half of the beef cubes, turning them over until all sides are nicely browned, 5 to 10 minutes. Remove the browned beef to a plate, and repeat with the rest of the canola oil and beef cubes. Transfer to plate, and cook and stir the onion in the hot skillet until translucent, about 5 minutes.
Whisk together beef broth, mushroom soup, and skim milk until smooth. Return the beef cubes to the skillet together with any juices on the plate, add the mushrooms, and pour the broth 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 cornstarch and water to make a smooth paste, and stir into the gravy. Simmer about 2 minutes to thicken.
