Preheat oven to 400 degrees fahrenheit (200 degrees Celsius).
Heat oil in a large skillet over medium heat. Add onion, carrot and celery, stirring occasionally until onion is glossy and vegetables begin to soften, about 7-10 minutes. Add garlic, stirring constantly for 1 minute.
Add garlic powder, onion powder, oregano, salt, pizza sauce, veggie ground and cooked grain along with 1 cup of water. Stir and simmer 3-4 minutes, until liquid mostly absorbed. Turn off heat. Stir in parsley. Taste and adjust spices or salt if necessary.
Place peppers in a high-rimmed baking dish and stuff with veggie mixture. Cover dish tightly with foil and bake for 45 minutes. Remove from oven and add cheese if desired. Place back into the oven and bake for another 10-15 minutes until peppers are starting to brown and cheese is melted.
Remove from oven and let sit for 5 minutes before serving.
Peppers will keep well, covered, in fridge for 3-4 days. Either dice up and reheat in skillet or rewarm, individually wrapped in foil and placed on a baking sheet, in 400 degree oven for 20-25 minutes until warm.