Bring a large pan of water to the boil and cook the penne as instructed on the pack. Meanwhile heat 1tbsp oil in a frying pan and cook the onion and mushrooms for a few minutes over a high heat until softened and crisp at the edges. When the pasta is cooked, drain, reserving a little of the cooking water. Toss with the mushrooms and onion mixture along with the cheese and chives. Loosen with a little of the water if needed.