Preheat oven to 350°. Cut peppers in half and scoop out seeds and veins. Lay upright in baking dish, cover with foil and pre bake while oven is preheating, approximately 15 minutes.
Saute onion and garlic in olive oil at medium heat just until softened. Add diced pepper, hot pepper if using, chopped mushrooms, diced peeled eggplant or summer squash (no need to peel summer squash).
Once all vegetables are softened, spread to edge of pan and add sausage meat. Brown meat, breaking up chunks until almost cooked. Stir all filling ingredients together. Taste and season with salt and pepper if needed.
Stir shredded cheese and parsley into stuffing mixture.
Remove peppers from oven and pour out any liquid that has formed in the bottom of peppers. Add stuffing, pressing well into the bottom and heaping on top. Cover with same foil and bake 15 minutes.
Remove foil, add breadcrumbs and change oven setting to convection bake or roast. Cook another 15 minutes or until top is browning.