FISH SOUP BASE
Do not remove the skin from the rock fish. Clean them carefully, gut the larger fish and rinse well.
Clean the leeks and cut them into large pieces. Peel and roughly chop the onions. Peel and lightly crush the garlic cloves. Wash the parsley and fennel thoroughly and let them drain. Crush the tomatoes by hand or with a utensil.
Heat olive oil gently in a large pot. Add the prepared vegetables together with the bay leaf, orange zest and chilli. Cook slowly over low heat for about 15 minutes, stirring occasionally.
Add the rock fish to the pot, season with salt and pepper, and let everything simmer gently so the flavours can develop. After about 15 minutes, pour in approximately 3 litres of boiling water.
Continue cooking over low heat for another 10 minutes.
Remove the fennel stalks and the orange peel. Mash the remaining contents using a coarse masher, then strain the mixture through a fine sieve or chinois to obtain a smooth soup base.
Add saffron, adjust seasoning if needed, and set the soup aside.
SAUCE AND CROUTONS
Peel two garlic cloves, remove the germ and finely chop them.
Place the mustard and egg yolks in a bowl and mix until smooth. Add the garlic, then slowly whisk in about 25 cl of olive oil until a thick sauce forms. Keep refrigerated until serving.
Preheat the oven grill. Slice the bread thinly and brush lightly with olive oil. Toast the slices on both sides until golden. Once removed from the oven, rub the warm croutons with a peeled garlic clove.
FINAL STEPS
Clean and gut the remaining fish, then cut them into evenly sized pieces.
Peel the potatoes, wash them and cut into cubes. Cook them in a pot for about 30 minutes, covered with an equal mix of fish stock and water. Season with salt, then set aside.
Bring the remaining fish stock to a gentle boil. Add the fish pieces, starting with those that have firmer flesh, and poach over low heat for 6–10 minutes.
Arrange the cooked potatoes on a large serving dish and place the fish pieces on top.
Place the garlic croutons in a soup bowl and pour the hot bouillabaisse over them. Serve the soup alongside the fish and potatoes.