Cook the pasta in lots of boiling water following pack instructions. Meanwhile, cook the leeks in the stock until really soft. Add the lemon zest and juice, fish, crème fraîche and parsley and take the pan off the heat.
Drain the pasta, holding back a little of the cooking water, then add pasta to the pan. Mix well and season with lots of black pepper, then add a little of the cooking water if it looks a bit thick.
Divide between four bowls and serve immediately with extra lemon, if you like.