Chop the onions very finely and brown them in a cup and half of oil (half olive oil, half nut oil), turning frequently, cooking time: 3/4 hr.
At the same time cook a court-bouillon with 2 l of water, 2 large whole onions, a sprig of bay and of thyme, 2 cloves of garlic, Espelette pepper, and sea salt, adding to this the fish heads.
Prepare the fish, clean them, remove the dorsal bone from the gurnards, cut them into 1 to 2 cm slices, cut the eels into halves and place on a plate to one side.
Salt everything.
Clean the mussles, open them and keep the juice.
Finely chop the tomatoes and add the onions, as well as the meat recovered from the heads.
Season with pepper and sea salt, then cook for 10 minutes. Add diced potatoes.
When 3/4 cooked (c. 20 minutes), add the mussels and the court-bouillon taken from the heads; stir continuously.
Then add the slices of gurnard and monkfish. If there is not enough water add the juice from the mussels; the upper slices, those of the monkfish, should only just be submerged. They should cook for 10 minutes. Stir continuously, not with a utensil but by moving the pot by its handles.
To re-heat the next day, the mixture should be boiled well but for no longer than 10 minutes, otherwise the fish will disintegrate.
When serving, take deep ladlefulls, so that everyone receives some of each layer.
