Heat 2 tablespoons olive oil with cooking spray in a large non stick skillet over medium heat for a few seconds. Add onions, stir and lower heat.
Cook until onions start to brown, stirrring occasionally. Add 3 garlic cloves and a few drops of water to mixture, it mixture will start to turn brown.
Cook another few minutes, scraping the bottom of the pan so that nothing sticks.
Add carrot, mushrooms, cabbage, celery, and peppers and continue to saute on low heat until vegetables are soft.
Season and gently mix with rice.
Cut off tops of red peppers with a small, sharp knife and hollow out. Rinse well and shake to dry.
Stuff loosely with rice mixture and set aside.
Meanwhile, in a shallow, 6-quart pot, combine all the sauce ingredients and bring to a boil over medium heat, constantly mixing with a wooden spoon. Taste and correct seasoning. Add more water if necessary.
Carefully place stuffed peppers in pot so that they are all standing up. Spoon sauce on top of each pepper.
Cook covered for about 1 1/2 hours or until peppers are soft.