Fry the mushrooms in the butter for about 3 mins, then remove from the heat and reserve a good spoonful. Add the flour to the rest, then blend in the milk and return to the heat, stirring all the time to make a thick sauce. Stir in the cheese, season to taste, then leave to cool.
Heat oven to 200C/fan 180C/gas 6. Butter 8 x 150ml soufflé dishes and line the bases with baking paper. Stir the egg yolks into the soufflé mixture, then whisk the egg whites until stiff before folding in carefully. Spoon into the soufflé dishes and bake in a roasting tin, half-filled with cold water, for 15 mins until risen and golden. Leave to cool (they will sink, but they are meant to). You can make the soufflés up to this stage up to 2 days ahead. Cover and chill.
When ready to serve, turn the soufflés out of their dishes, peel off the lining paper, then put them on a baking sheet lined with small squares of baking paper. Top each soufflé with 1 tsp crème fraîche and a little cheese, then scatter with the reserved mushrooms. Bake at 190C/fan 170C/gas 5 for 10-15 mins until slightly risen and warmed through. Sprinkle
with chives and serve.