Preheat a large frying pan, over a med to high heat. Fry the mushrooms and black pepper with 50 grams butter for about 3 minutes. Remove the mushrooms from the pan and set them aside.

Reduce the heat to meduim to low. Add the remainder 50 grams of butter and let it melt. Now add the flour whisking it with the butter until it forms a light dough, cook for 30 seconds.

Begin adding the cream bit by bit, whisking continuously so that it doesn't get lumpy. Do the same with the milk. Simmer for 2 minutes whisking constantly. Add a bit more milk if it's too thick.

Add the mushrooms and salt, simmer for 3 minutes, whisking constantly. Check for salt and pepper and add more if you need to.