Bring a large pot of water to boil. Add cabbage leaves and cook until just tender, about 2-3 minutes. Cool in cold water, drain well, pat dry, and reserve. To make filling, heat oil in a skillet set over medium heat. Add onion, peppers, and mushrooms, and cook until tender and any liquid exuding from the mushrooms has evaporated. Remove from heat and cool to room temperature, and then mix in remaining filling ingredients.
To make the sauce, heat oil in a skillet set over medium heat. Add onions and garlic and cook until tender. Add remaining ingredients and gently simmer 10 minutes. While this occurs divide and place filling in the centre of each cabbage leaf. Fold over sides and then tightly roll. Place cabbage rolls in a casserole or baking dish. Pour over sauce. Cover and bake in a preheated 350 degrees F oven 30-40 minutes.
Note: Cabbage contains a good amount of Vitamin C and some vitamin A. If the cabbage leaves are small, simply use more of them to use up the filling.