Step 1Wipe the mushrooms with a damp cloth to remove dirt. Trim the stems to remove any moldy tips. Cut the matsutakes in half lengthwise. Place them on a square of aluminum foil.
Step 2Brush the matsutakes with the sake. Fold the foil into a tightly sealed packet.
Step 3Heat a grill pan or a grill until very hot. Place the packet on the hot grill and cook for 5 minutes. Turn the packet over and grill 5 minutes more.
Step 4Open the foil carefully, as there will be steam from the packet. Sprinkle with fleur de sel to taste and serve immediately.