Heat 2tsp butter in a large saucepan and sweat one third of the onions, shallots and leeks over a low heat. Add the mussels, clams, one bouquet garni and white pepper, together with the white wine, and agitate the pan to allow all the mussels to receive heat on the bottom layer. When all the mussels are opened (discard any that are still closed), drain off the liquor into a bowl and reserve, then remove the shellfish meat from the shells, removing any stray beards. Set aside. Heat the remaining butter in a large saucepan and sweat off the remaining leeks, onions and shallots with the garlic, chopped tomatoes, diced carrots, celery and fennel. Add the other bouquets garni and cook for 5 minutes. Add the stock and reserved shellfish liquor and bring to the boil. Keep some clams and mussels for garnish, and put the rest with the saffron into the pot to simmer for 20 minutes. Remove the bouquets garni, then add the double cream and pour into a processor. Whiz until smooth. Adjust the seasoning and return to the saucepan to heat through gently. Serve with a sprinkling of chopped chervil and the reserved mussel and clam garnish. Tip: fatten the mussels up when you get them home by placing them in some cold salted water (you will need about 4 handfuls of salt per 2-gallon bucket of cold fresh water). Stir in some plain flour or oatmeal (that has been whizzed in a blender for a few seconds). If left for a few hours or even overnight, the shellfish will filter feed which has the effect of allowing them to clean themselves and fatten themselves up for cooking!