Heat a barbecue or ridged grill pan.
Remove the stalks from the mushrooms, and in the case of the flats, peel the caps; simply wipe ceps with a damp cloth. Stems from flats are useful in stocks; stems from ceps should be cut lengthways and grilled along with the caps.
Grill the caps top side down until water collects in the hole where the stalk was. This will take 8–10 minutes. Turn the mushrooms and grill for 2 minutes on the other side. Transfer to a serving dish, and scatter with gremolata, salt, pepper and olive oil. They will give out copious and delicious juices, so sop them up with bread.