Preheat your oven to 375°F (190°C).
Start by preparing the bell peppers. Cut the tops off the peppers and remove the seeds and membranes from the inside. Rinse them under cold water.
In a large skillet, cook the ground meat(or meat substitute) over medium heat until it's browned and cooked through. Break it into small pieces as it cooks. If using meat, drain any excess fat.
In the same skillet, add the diced onion, celery, carrots, and minced garlic. Sauté for about 5 minutes, or until the vegetables begin to soften.
Add the cooked rice, diced tomatoes, Italian seasoning, salt, and black pepper to the skillet. Mix everything together and cook for another 2-3 minutes to blend the flavors.
Stuff each bell pepper with the meat and rice mixture, packing it in firmly. Place the stuffed peppers in a baking dish.
In a small bowl, mix together the ingredients for the tomato sauce: tomato sauce, dried basil, dried oregano, garlic powder, salt, and black pepper.
Pour the tomato sauce evenly over the stuffed peppers.
Cover the baking dish with foil and bake in the preheated oven for 35-40 minutes, or until the peppers are tender. Remove the foil for the last 5-10 minutes of baking to allow the tops to brown slightly.
Once the stuffed peppers are cooked, remove them from the oven and let them cool for a few minutes before serving.