Empty the contents of the cheese sauce sachet into a jug and fill to 400 ml with boiling milk and stir for 1 min.
In a large pan, brown the red onion, garlic, mushrooms and leeks in oil.
Add a splash of white wine (optional).
Add the sachet of seasoning mix, 600 ml hot water and pasta to the mushrooms.
Bring to the boil, stirring occasionally.
Reduce heat, and simmer for 10 min and pour into a 2-litre oven dish.
Pour cheese sauce over and sprinkle with mozzarella cheese.
Grill for 10 min or bake for 20 min at 180°C .