Wash and dry the mushrooms and carefully remove the stalks. Wash and finely dice the courgette and peppers into small pieces.
In a pan heat 2tbsps of the oil and then add the garlic and fry for a minute or two. Add the vegetables, and season with salt and pepper and cook for 5 minutes.
Preheat the oven to 150°C, gas mark 5.
With the remaining oil grease a large (or 2 large) flat dishes and arrange the mushrooms, hollow side upwards. Fill the mushrooms with the vegetable mixture and cook in the oven for 35-40 minutes. Serve hot, garnished with sprigs of rosemary.