▢600 g / 1.2 lb scotch fillet steak / boneless rib eye (Note 1)
▢2 tbsp vegetable oil , divided
▢1 large onion (or 2 small onions), sliced
▢300 g / 10 oz mushrooms , sliced (not too thin)
▢40 g / 3 tbsp butter
▢2 tbsp flour (Note 2)
▢2 cups / 500 ml beef broth , preferably salt reduced
▢1 tbsp Dijon mustard
▢150 ml / 2/3 cup sour cream
▢Salt and pepper