Cook the pasta in plenty of boiling salted water until tender. Meanwhile fry the onion and garlic in the oil for 5 minutes until softened.
Add the tuna and mushrooms and 150ml (¼ pint) of water and cook gently for 10 minutes or until the pasta is cooked. Stir in parsley and season to taste.
Drain the pasta and return to the pan. Tip in sauce and toss well together. Divide into bowls and serve with a good sprinkling of black pepper on each.