Peel the garlic cloves and bring them to a boil in a small pan of water. Drain them and bring them to the boil again in fresh water. Simmer for 5 minute then drain.
Put the mushrooms into a baking dish and smear with the butter. Season with salt and pepper. Tuck the boiled garlic cloves all around the mushrooms and scatter with thyme.
Roast in a preheated Oven 200 C, Fan 180, Gas 6 for 20 minutes, basting from time to time to prevent the mushrooms from drying out.
Serve the mushrooms with their cooking juices and the garlic, accompanied by a few salad leaves and some bread to mop up the juices.