Trim the end from the cabbage and peel off the outer leaves if they are damaged. Carefully remove the core from the cabbage, but leave the cabbage whole, by making a kind of cone shape into the base (see photo above).
Place the cabbage in a pot large enough to hold it covered, base down, and cover with water. Bring to a boil then simmer for around 3 - 5 minutes until the leaves are tender enough to peel off and seem relatively flexible. If they don't feel tender enough, cook slightly longer. When ready, drain and allow to cool slightly then carefully peel the leaves off and let them cool.
Pick out the eight best leaves for filling then set aside the others - you will use most to help line the pot you cook the cabbage rolls in. Carefully use a paring knife to trim back the thick part of the stem on the outside of each of the eight leaves for stuffing so that the remaining stem is about as thick as the rest of the leaf.
Meanwhile, finely dice the onion and garlic. Warm the oil in a skillet over a medium heat then soften the onion. After a couple minutes, add the garlic and soften that as well then remove from heat once all are softened and allow to cool.
Place the ground pork in a bowl. Once cooled, add the onion and garlic mixture to the pork, along with the cooked rice, marjoram, salt and pepper. Mix everything well so that everything is well mixed and distributed.
Take approximately ¼ cup of the mixture and place it on top of one of the cabbage leaves, fold in both sides then roll it from the base to form a thick log. Don't make it too tight, just snug, to try to avoid it bursting during cooking. Repeat with the rest of the leaves and filling mixture.
Line a pot large enough to hold all of the cabbage rolls (ideally in a single layer) with some of the leftover/torn leaves - this is to help prevent the rolls burning as they cook. Lay the rolls on top, placing them relatively snug together. If you need to make a second layer, that's fine, but ideally one is best. If you like, add any additional leaves on top to help keep steam in as they cook.
Pour over the stock - it should just about cover the rolls - then cover and place on the stove on a medium heat. Bring to a simmer, reduce the heat slightly, and simmer for around 35 - 40 minutes until the rolls feel slightly firm since the filling is cooked. Set aside a few minutes as you make the sauce. Don't discard the cooking liquid.
Warm the butter in a skillet or wide pot and once gently foaming, add the flour. Stir to mix into a smooth paste for a minute. Drain off some liquid from cooking the cabbage rolls and add around ½ a cup, a little at a time, to the flour-butter paste to form a smooth sauce. Cook a minute then add the tomato passata, mix and allow to cook a couple minutes.
Drain the cabbage rolls from the cooking liquid and serve topped with the sauce. Traditionally served with mashed potato.