In a large pan of salted boiling water, cook the pasta according to the packet instructions. Drain well, reserving a few spoonfuls of the cooking water.
Meanwhile, heat the oil in a large frying pan over a medium-high heat. Add the mushrooms and toss around in the hot pan for a couple of minutes, until they start to brown. Stir in the leek and lower the heat to medium. Cook gently for about 5 minutes, until sweet and soft.
Add the drained pasta to the mushrooms and leek, and stir together with the Alfredo Pasta Sauce. Serve, scattered with parsley and plenty of Parmesan.