In a casserole containing the oil, put the chopped mixture of onion and leeks; leave to become golden and pour in the wine.
Add the tomato, saffron, thyme, bay leaf and a pinch of salt. Leave to simmer over moderate heat, until the wine has half evaporated.
Add the mussels, leave to gain flavor for some seconds and transfer the preparation to a concave serving plate to cool. Leave to rest in a cold place, sprinkle with a handful of chopped parsley and serve.
If you prefer, you can remove the mussels from their shells before adding them to the casserole, and serve them shelled.