Peel and dice the onion. Clean the leek, cut open, wash and cut into rings.
Heat the oil in a saucepan, peel the garlic, chop it up and add it. Stir in the onions and leek and sauté. Chop the tomatoes a little and add them with the juice, mash a little if necessary.
Pour in the white wine and stir in the seafood. Season with salt, pepper, saffron and chilli flakes. Cover and simmer for 10 minutes, stirring occasionally.
In the meantime, wash and dry the fish fillet in cold water, dice bite-sized and season with salt and pepper.
Refine the seafood pot with sherry, stir in the fish cubes, set the stove to the lowest setting and let everything rest for another 7 minutes.