Fry off the sliced mushrooms and chopped onion in some olive oil until soft but not coloured. Drain off the oil but keep to one side.
Cut the peppers in half through the stalks and scoop out the seeds. Put the peppers on a baking sheet and drizzle with the oil from the mushrooms. Sprinkle with salt and pepper and bake in the oven for 20-25 minutes until the peppers are just tender.
Cook the rice and fluff with a fork. Stir in the mushrooms, onions, peas and sweetcorn, cheese and parsley. Spoon into the pepper halves, return to the oven and bake for 15 minutes until the cheese is golden brown.