1. Pre-heat the oven to 350 degrees.
2. Core the cabbage and put into a pot of salted boiling water. Peel off cabbage leaves as they separate from the cabbage. Cook leaves until soft. Remove from the pot to a pan to cool.
3. Heat grape seed oil in a pan. Put in onions, and cook on medium heat for 10 minutes, or until onions are soft and sweet.
4. Add celery, fennel, carrots, mushrooms, zucchini, and garlic, and cook for 10 minutes.
5. Add thyme, tomato paste, and balsamic vinegar, and mustard powder and cook for 5 minutes.
5. Add stock and cooked rice. Remove from heat. Add salt and pepper. Place mixture in a bowl, and cool.
6. Line the bottom of a roasting pan with the small Port blanched cabbage leaves.
7. Scoop some of the mixture into a large cabbage leaf and roll. Place the rolled cabbage on the bed of cabbage in the roasting pan. Repeat until all of the leaves are filled.
8. Season the tomatoes with salt and pepper. Puree and pour over top of the rolled cabbages.
9. Bake for an hour.