Bring a large pot of lightly salted water to a gentle boil. Submerge the whole cabbage head into the boiling water, cover the pot with a lid, and allow it to cook just until the cabbage leaves soften enough to peel off easily, approximately 3 minutes. This brief blanching keeps the leaves tender yet firm enough to handle. Carefully remove the cabbage from the pot and set it aside to cool for about 10 minutes, so you can work with the leaves comfortably without burning your hands. Using a sharp knife, gently separate 18 whole cabbage leaves, making sure to cut out any thick, tough ribs from the center so they roll smoothly without tearing. Reserve these whole leaves for wrapping, and finely chop the remaining cabbage core and bits, spreading them evenly to form a bed at the bottom of your casserole dish. This not only adds flavor but prevents the rolls from sticking. Next, melt butter in a large skillet over medium-high heat. Add the finely chopped onion and sauté, stirring frequently, until it becomes soft, translucent, and fragrant, about 5 to 10 minutes. Allow the cooled onions to rest before mixing. In a large mixing bowl, combine the sautéed onions with ground beef, ground pork, uncooked rice, minced garlic, salt, and black pepper. Stir thoroughly until all ingredients are well blended. This mixture will be the savory filling nestled inside each cabbage leaf. Preheat your oven to 350°F (175°C) to prepare for baking. Lay a generous half-cup of the meat mixture onto each cabbage leaf. Fold in the sides neatly and roll the leaf tightly, forming a neat envelope that encases the filling securely. Place the rolled cabbage parcels in a single layer atop the chopped cabbage in your casserole dish. Lightly season the rolls with a pinch of salt and freshly ground black pepper for added flavor. In a bowl, whisk together tomato puree and the contents of the tomato cans (tomato juice) along with ketchup until smooth and well combined. Pour this vibrant tomato sauce evenly over the cabbage rolls, ensuring they are fully bathed in the rich liquid. Cover the casserole dish tightly with aluminum foil to keep moisture in during baking, which helps the rolls cook evenly and stay tender. Bake in the preheated oven for about 1 hour, or until the cabbage is tender and the meat filling is cooked through. Tip: Serving your Golabki with a dollop of sour cream adds a delightful creamy tang that complements the savory tomato sauce perfectly. For extra tender cabbage rolls, make sure not to overboil the cabbage initially; the leaves should be pliable but not falling apart. Leftovers taste even better the next day as the flavors continue to meld.