Wash and cut the mushrooms. Dice the onions and fry them in the butter with the mushrooms. Pour in white wine, season with salt, and let simmer in a closed saucepan for 10 minutes.
Mix the creme fraiche with the egg yolk and mustard, season with salt and pepper.
Cut pork fillet into medallions, fry in hot fat for a good 2 minutes on each side, season with salt and pepper.
Mix the mustard cream with the braised mushrooms and season with tarragon, salt and pepper.