Melt the butter on low heat in a small sauce pan. Add the shallots and cook, stirring frequently, for five minutes. The shallots should not brown, otherwise they will turn hard and bitter.
Pour in the wine and add half of the chopped dill. Turn up the heat to medium and boil the liquid until about 1/4 cup remains.
Turn down the heat and stir in the crème fraîche and mustard. Warm through and season to taste with salt and pepper. Stir in the remaining dill just before serving.
Makes about 1 cup of sauce.