Peel garlic, chop finely and bring to the boil with oil, lemon juice, peppercorns, bay leaf, thyme, 1/2 teaspoon salt and 1/8 litre water. Simmer at low heat for about 20 minutes.
Wash and clean the mushrooms and cut into thick slices. Add the mushrooms to the marinade, bring to the boil briefly and leave to cool.