Ask your fishmonger to fillet the fish for you, or do this yourself if you’re able. Save the fish carcass. Add the fish bones to a pot with 2 l of water. Add the tomato purée to the water along with the basil leaf and a teaspoon of salt. Cook on low heat for about 1 hour. Filter the liquid and top up with more fresh water to cook the gnocchi in.
Make the sauce for the gnocchi now. Finely slice the onions. Add the onion and the tomato purée to a large pan together with a piece of dried chili and the lemon zest. Tear in some basil leaves. Add a small amount of oil and cook on low for 7 to 8 minutes.
Next add the wine, raise the heat and reduce.
Add the fish and cook until tender enough to break with a fork. Remove from the heat and set aside.
Cook the gnocchi and gently remove them from the boiling water with a slotted spoon.
Add the cooked gnocchi to the pan along with a ladleful of the pasta water. Toss gently and serve.