First of all, you need to prepare the mussels so begin by discarding any that have broken shells or any that don't close when given a sharp tap with a knife.
Then, under cold running water, scrub the mussels, removing any barnacles, and pull out the little hairy 'beards'. After you have cleaned each mussel, place it straight into a bowl of clean water and when they're all in, swirl them around to get rid of any lingering bits of grit or sand. Drain them in a colander and they are now ready to be cooked.
Next, pour the wine into a large, wide-bottomed pan and when the wine comes to the boil, put two tablespoonfuls into a cup or small bowl. Stir in the saffron and leave it to infuse. Next, tip the mussels into the pan, cover with a lid and leave on a high heat for 4-5 minutes. Have ready a colander lined with fine muslin or a clean, fine tea towel, set over a large bowl, and when the mussels are ready, tip them into the colander and leave them to drain. Now rinse out the pan (in case there is any fine grit left from the mussels) and melt the butter in it and then, over a low heat, soften the chopped shallots in the melted butter for about 5 minutes or until lightly golden.
Meanwhile, remove the mussels from the shells, discarding the shells as you go, and make sure you throw away any mussels that have not opened during cooking. Now tip the reserved mussel cooking liquor into a jug. Next, add the garlic to the softened shallots and cook for a further 30 seconds. Follow that with the flour and, using a small pointed wooden spoon, stir quite vigorously to make a smooth, glossy paste. Now increase the heat to medium and begin to add the reserved mussel cooking liquor, a little at a time, still stirring. When all the liquid has been added, bring the mixture up to simmering point and cook on the lowest possible heat for 1 minute, stirring all the time. Next, add the saffron (and its soaking liquid), the milk and the crème fraîche.
Bring the soup back up to a gentle simmer, switch to a balloon whisk and whisk until you have a smooth, creamy mixture. Now add the shelled mussels, two tablespoons of chives and heat gently for 5 minutes. Season and serve in warmed bowls, garnished with the remaining chopped chives.