Step by step
Fill the upside down mushroom caps with the gratin mixture, and place in a roasting pan or baking dish. Place in the oven for 20 minutes at 180 °C and then for the last 10 minutes under the grill.
Tips
Do you fancy making a really successful mushroom gratin? Then choose fairly large mushrooms that are also fairly firm, so that they can easily hold the filling.
Trivia
If you don’t want to throw away the stems of your mushrooms, keep them and use them sautéed to make a lovely pasta sauce.