Soak the porcini mushrooms in water for at least half an hour then drain, retaining the soaking liquid.
Make up the polenta according to packet instructions using the soaking liquid (make up volume with stock or plain water if necessary).
Meantime fry the onions, mushrooms and garlic in olive oil, then add to the polenta.
Cut the top off the peppers and deseed, fill with the polenta mixture.
Bake for about 45 minutes at gas mark 6, adding grated cheese to the top for the last 5 minutes if desired.