In a liquidiser, chop the whole tomatoes with juice and garlic until chunky. In a medium size pot, heat oil on medium high heat. Put onion and mushroom in and saute for about 5 minutes.
Add liquidised tomatoes, passata, tomato puree, chopped green peppers, oregano and basil. Bring to the boil, then lower to medium low, cover and stir every now and again. Cook and reduce about 1 to 2 hours or to your liking.