Beef Stroganoff with margarine

1 pound sirloin steak cut into thin strips 3 tablespoons flour, all-purpose divided 1/2 teaspoon salt 4 tablespoons butter or margarine 3 ounces mushrooms,[...]

1/2 pound beef chuck steak 1 tablespoon butter or margarine 1/2 tablespoon vegetable oil 1/8 pound mushrooms sliced 2 tablespoons onion finely chopped 1/4[...]