Melt the butter substitute in a saucepan and then gently fry the chopped onion and garlic for 1-2 minutes. Add the rice and stir to coat in the butter. Stir in the red wine and bring to the boil before adding the stock. Simmer gently for 12-15 minutes stirring occasionally. Meanwhile, prepare the steak. Drizzle half of the oil over the steak and season generously with salt and black pepper. Heat a griddle pan till hot and then cook the flank steak for a few minutes on both sides till medium. Remove the flank from the pan and throw the mushrooms into the pan juices. Cook for 2 minutes before adding the sherry or Madeira wine and reduce. Finally, add the mascarpone and chopped parsley to the risotto and season to taste with salt and pepper. Spoon the risotto onto serving plates. Slice the steak and place alongside with the oyster mushrooms. Drizzle with the pan juices and serve.