Open the oysters, reserving the liquor. Remove the oysters from their shells and arrange in a shallow flameproof serving dish.

Clean the button mushrooms by wiping them with paper towels; chop them very finely. If using chanterelles, rinse them briefly under cold water, drain and pat dry. Chop into small pieces.

Melt the butter in a skillet, add the shallots and sauté for 1 minute. Stir in the mushrooms and lemon juice and cook for I minute more, then add the reserved oyster liquor with black pepper to taste.

Stir in the parsley and crème fraîche. Spoon some sauce over each oyster. Sprinkle with a little grated Gruyère. Place the dish under a preheated hot broiler until the cheese bubbles. Serve at once.