Make slits at the base of the cabbage or cut out the entire core. Fill a large saucepan about 1/3 full of water. Bring to the boil, then add the whole cabbage to the pan. Cook for 15 to 20 minutes, turning occasionally. Drain and cool, then cut the leaves off the cabbage one by one, trying to keep them intact. Cut out the centre of the large leaves where it is too tough or thick.
Cook rice according to packet instructions.
Heat 2 tablespoons of the oil in a frying pan, add one of the chopped onions and cook till soft.
In a large mixing bowl, combine the cooked onions with the mince, rice, salt and pepper. Mix well with your hands till combined.
Place 1 large tablespoon of meat filling at the base of a cabbage leaf. Fold over like an envelope, then roll to the top edge of the leaf. Repeat with remaining cabbage and filling.
Heat remaining oil and fry cabbage rolls a few minutes on each side, till browned. Transfer to a casserole dish along with the onion and carrot sprinkled on top.
Mix sour cream and tomato puree. Whisk in water, then season with salt (you can add your favourite dried herbs at this point, if desired). Pour sauce over the cabbage rolls.
Cover and cook over a medium low heat for 45 minutes.