First prepare the mushrooms: detach the stalks if necessary, and cut off and discard the earthy ends. Wipe the caps with a damp cloth, but do not wash or immerse in water - which they absorb like sponges. Peel the stalks, but not the caps. If very large, slice into pieces.
Make the garlic butter, cut the butter into small pieces. Peel, crush and finely chop the g