For the sauce, add the butter to the pan along with the finely diced shallot. Add in the mushrooms. Fry for a few minutes until the onions are translucent but not browning. Add the garlic, crushed. Stir for a further minute or so until you can smell the garlic. Reduce the heat and pour in the cream. Mix well. Season with salt and pepper and allow to reduce very slightly so that the cream thickens a bit and is slightly sticking to the spoon.