Cook the pasta in salted water. Chop mushrooms, bbq sausages and onion, fry in a little bit of olive oil. Add the bechamel sauce and extra milk, and stir through. Stir sauce mixture into cooked pasta. Place in oven dish, top with cheese. Set oven to 180 degrees. Bake for 20 - 30 minutes, until cheese is melted and browning.