Heat the oil in a medium frying pan and add in the leeks and mushrooms. Fry on a medium heat for 5 minutes until the leek has wilted and the mushrooms have softened. Add in the garlic and cream and stir. Cook for 1 minute. Add in half the cheese and stir.Transfer the gratin to a heat-proof dish and sprinkle with the remaining cheese and the black pepper. Grill until the cheese melts and starts to go golden.Carefully remove from the grill, sprinkle with the parsley and serve.