Clean and cut the tops off from the peppers. Gently insert a spoon or knife into the pepper to widen the pepper opening a bit. Set aside 1 or 2 peppers to dice up and saute in a hot pan with olive oil, onion, garlic, tomato, eggplant and parsley. If I had to guess I would say 3-4 heaping tablespoons. If you are using prosciutto/dried sausage add it now as well. Cook down over medium heat until the vegetables are softened but not mushy.
In a separate bowl combine the breadcrumbs and the cheeses. Pour the hot vegetable mixture and the frozen peas into the bowl of breadcrumbs. Mix together gently and then add one egg and mix together some more. You should now have a cheesy breadcrumb stuffing/filling.
Pre-heat oven to 400 degrees F. With a spoon take the filling and stuff each pepper to the top. Pack in the stuffing as best you can. Grease (with olive oil) a large baking sheet. Place the peppers on the baking sheet and roll them around in the oil to coat all the sides. Place them into a pre-heated 400 degree F oven and roast until they begin to brown and soften. Each oven is different but I would say it takes approximately 15-20 minutes. Keep an eye on the pepper and turn them as needed.