Satrt with the crumble : cut the butter into chunks.Tip the flour, cheese and breadcrumb in a large bow. Add the butter, rub into the flour using your fingertips to make a light breadcrumb texture.
Bring a pan of salted water to boil, add the leeks and cook for 3/4 minutes, drain. Cut the cod in medium size pieces. In a large baking dish put the cod, the chorizo chunks, the leeks, the cream, the lemon zest, salt and peper, combined. Sprinkle the crumble topping over the top and the hazelnuts.