Skin the tomatoes, remove the pips and cut the flesh into small cubes.

Clean the mussels, removing their beards if necessary and discarding those who do not close even after being tapped lightly. Thoroughly rinse the remaining mussels under running water.

Wash the leek and thinly slice the white part only.

Peel and finely dice the 3 garlic cloves, sprinkle over some salt and crush into a paste using a broad knife.

Heat the oil and butter in a wide saucepan or wok and sauté the leek and garlic in it until translucent. Add the saffron, herbs and chili pepper and cook for another 1 to 2 minutes.

Douse with the white wine and bring to the boil, then after 2 minutes add the tomato cubes and 250 ml of water. Cover and simmer over a low heat for 20 minutes.

Add the mussels to the soup and cook in the covered saucepan until they have all opened. Discard any mussels that haven't opened after 5 minutes.

Take out one third of the mussels, remove them from their shells and return the flesh to the soup. The remaining mussels are left whole in the soup.

Season to taste with salt and pepper and serve immediately.