Scrub the mussels thoroughly, debeard them and rinse them well. Discard any that are damaged or remain open.
Divide the mussels between two large saucepans, then add half the wine, 1 bay leaf, 1 sprig of parsley and 1 slice of lemon to each pan. Cover and cook for 6-8 minutes over a high heat, shaking the pans frequently, until all the mussels have opened. Do not overcook them or they will be tough. Discard any that do not open.
Meanwhile, place a large colander over a large bowl and line it with muslin or kitchen paper. As soon as the mussels have opened, pour them into the colander to drain and cool, reserving the liquid. When the mussels are cool enough to handle, remove them from their shells and put them into a bowl.
Bring a large pan of salted water to the boil and cook the tagliatelle according to the instructions on the packet.
Meanwhile, pour the strained mussel liquid into a saucepan, bring it to the boil, then let it boil until reduced to 300 ml (10 fl oz), pouring it into a measuring jug from time to time to check.
Blend the cornflour with 1 tablespoon of cold water and stir it into the reduced mussel liquid. Add the saffron, if using, and return to the boil, stirring continuously. Then reduce the heat, add 2 tablespoons of the dill and let the sauce simmer for 5 minutes for the flavours to infuse.
Gradually whisk the fromage frais into the sauce, then add the mussels and heat them through over a medium heat. Do not let the sauce boil or it will curdle and the mussels will be tough. There is no need to add salt as the mussel liquid has a salty flavour.
Drain the pasta and stir it into the sauce. Divide the pasta between four bowls, garnish with pepper and the remaining dill and serve.