Heat the butter in a saucepan, add the leeks and onion and a splash of water and cook over a low heat for 15 minutes until very soft. Stir in the garlic and cook for a further 1 minute. Add the cream and crumble over the Gorgonzola, then cook until the cheese melts.
Meanwhile, cook the pasta in a large saucepan of salted boiling water according to the pack instructions. Drain, reserving a little of the cooking water, and return to the pan. Stir through the sauce, adding a little cooking water to loosen if needed. Season well.
Spoon into serving bowls and serve topped with the walnuts.