Other suitable fish: salmón de roca, tollo, lisa, lenguado)
When you buy the congrio get the fishmonger to cut in slices and remove the skin. Put the slices of the prepared fish in a bowl and cover with the lemon juice, salt and pepper and leave for 2 hours. Meanwhile prepare the tomato sauce by frying 2 of the onions in 2 tbsp of butter. When they are well browned pour over the canned tomato sauce, mix well and season with salt and pepper. Cook for 5 minutes and set aside.
In another large pot fry the fish head in 1 tbsp butter until lightly browned. Add 2 cups of water, the chopped onion, the sliced carrot, bay leaf, salt and pepper. Simmer for 1 hour. Strain the stock and reserve the cooked carrot. In an earthenware oven dish put the tomato sauce, the strained fish stock, the lemon juice marinade and the wine. Put the pieces of sliced fish with 3 tbsp of grated cheese on top. Put the dish in a 350ºF (175ºC) oven and bake for 1 hour.
Beat the egg yolk with the cream or milk. Pour over the fish and taste for seasoning. Decorate with the cooked slices of carrot. Return to the oven for 10 minutes. Serve with chopped parsley on top.
This is a thick soup, almost like a stew. You can add some shellfish and fried chopped garlic with the onions if you like.
You can also ad cooked and sliced potatoes to the dish.