Wash the mussels thoroughly and remove the beards. Boil a little water in a saucepan, add the mussels, cover and steam for 4 minutes until opened. Discard any that remain closed. Preheat the oven to 220°C (200°C fan, gas 7).
Plunge the tomatoes into hot water, then into iced water, then drain. Peel off the skins, quarter, deseed and finely dice.
Open the mussels, remove the lids and place the half with the flesh in 4 small cast iron pans.
Peel and finely slice the garlic. Finely chop the olives and anchovies. Saute everything with the garlic in 2 tbsp hot oil in a pan. Add the parsley.
Pour in the white wine and allow to bubble. Season with salt and pepper.
Spoon over the mussels.
Sprinkle with the crumbs and drizzle with the remaining oil. Brown in the oven for 5 minutes and serve with bread if desired.