Finely chop the onion and cook it in a hot skillet with the oil over low heat until translucent. Add the rinsed rice and cook for a few more minutes, until the rice looks slightly translucent.
In a large bowl, combine the ground pork and beef, finely chopped smoked pork belly, the cooked onion and rice mixture, salt, black pepper, and thyme. Mix well until evenly combined.
Separate the sour cabbage leaves and rinse them well to remove excess salt. If they are very salty, soak them in water for 2 to 3 hours. Trim the thick cores from the leaves, and cut the thinner parts as needed to make them easier to roll. Finely chop any leftover cabbage, since it will be used for layering in the pot.
Take one cabbage leaf at a time and place about 1 tablespoon of filling near one end. Roll it tightly, folding the sides inward as you go to seal the roll. Repeat until all the filling is used.
Grease the bottom of a large pot with a little oil, then add a layer of chopped cabbage. Add a layer of smoked pork pieces or smoked bacon on top.
Arrange the cabbage rolls in layers, alternating with chopped cabbage and smoked meat, so that the final layer is chopped cabbage with bay leaves and a few thyme sprigs. Pour in the tomato passata, adding enough water to cover the rolls.
Cover the pot and cook over low heat for 3 to 4 hours, until the cabbage is very tender. If the liquid reduces too much, add a little hot water.
Serve the cabbage rolls hot and fragrant.