Melt the butter in a saucepan over a medium heat and cook the shallot for 1-2 mins. Stir in the flour and cook for 2 mins more, then add the white wine, stirring well to make a thick paste.

Slowly pour in the stock, whisking continuously, then whisk in the milk and bring to a simmer. Stir in the mustard and crème fraîche, then season well. Remove from the heat and stir in the dill just before serving.