Bring a large pan of water to the boil and add 2 level teaspoons salt. Cook spaghetti for 12 to 15 minutes until just tender.
Fry garlic and onion in 2 teaspoons oil till soft. Also add mushrooms if using. Pour on stock and sherry and boil rapidly for 3 minutes to reduce the liquid. Stir in tuna and 2 tablespoons parsley. Add salt and pepper to taste and stir in the cream. Adjust seasoning.
Drain spaghetti and toss with sauce. Garnish with remaining parsley before serving.