Cut off the upper part of the peppers, remove the seeds and blanch in the boiling salted water, drain.
Steam the onions, garlic, tomatoes and mushrooms in the oil. Season to taste with salt and pepper. Add the mozzarella. Pour the mixture into the peppers.
Put in a baking dish, sprinkle with 1 teaspoon of Parmesan per pepper, and bake in the oven for 10 minutes at 180 degrees. Also works in the microwave.
There may be some filling left over, if this is the case, just spread it around the peppers.