Preheat the oven to 180°C.
Remove the mushroom stems and finely chop them.
Arrange the mushroom caps on a lightly oiled baking tray and drizzle with a little olive oil.
Roast for 15 minutes until they begin to soften.
Meanwhile, place the chopped prawns, grated courgette, garlic, chopped mushroom stems, egg, chilli flakes, and herbs in a bowl.
Season with salt and pepper and mix well.
Remove the mushrooms from the oven and carefully pour away any liquid that has collected inside them.
Divide the filling between the mushroom caps, pressing it in gently.
Sprinkle with grated cheese, if using.
Return to the oven and bake for 5–8 minutes, until the filling is cooked through and the tops are lightly golden.
Serve warm with watercress or wild rocket salad and a few walnuts, or arrange on a platter and drizzle with balsamic glaze for an easy party appetiser.