Preheat the oven to 400 degrees Fahrenheit and bring a large pot of salted water to a boil.
Combine the ground beef, pork, parboiled rice, onion, garlic, salt, black pepper, thyme, and sage in a large bowl. Mix well.
Whisk the tomato sauce and chicken broth together in a medium bowl. Set aside.
Begin by discarding any wilted or loose leaves from the cabbage. Then, remove as many outer leaves from the cabbage as possible without tearing them. Transfer the leaves to the pot of boiling water and blanch them for 3 minutes, or until tender. Carefully remove the leaves using cooking tongs. Drain and set aside on a large cutting board or plate.
Next, place the whole head of cabbage in the pot of boiling water (ideally, it will be fully submerged) and boil for 3-4 minutes. Carefully remove the cabbage from the pot and use tongs to gently remove as many outer leaves as possible without tearing them. Return the head of cabbage to the boiling water and repeat.
Continue boiling and peeling off the outer layers of the cabbage using this method until the leaves are too small and crinkled to work with. Note - The core of the cabbage takes longer to heat through, so after you’ve removed several of the outer leaves, you may need to let the cabbage cook for a few more minutes to soften the next layer of leaves.
Once cool, trim any leaves with a thick center stem by making a V-shape cut using a sharp knife.
To roll - Lay the cabbage leaf flat. If using a leaf with the thick stem cut from the one center, overlap the cut parts slightly to close the gap. Spoon approximately 3 tablespoons of the beef mix and rice mixture onto the leaf. Roll it up by folding in the outer flaps then rolling the top over the sliced part to seal it as best as possible.
Repeat the process with the remaining cabbage and meat. Tip - Double up and use two leaves once you've used the bigger leaves.
Drizzle the olive oil into a 9x13-inch baking pan or casserole dish. Place the rolls into the pan, seam-side-down.
Pour the prepared tomato sauce and chicken broth mixture over the cabbage rolls, distributing it as evenly as possible. Tightly cover the baking dish with aluminum foil (this will trap any steam, preventing the cabbage rolls from drying out).
Transfer to the preheated oven and bake for 1 hour. After an hour, vent the foil and continue to bake for an additional 20-30 minutes. Once the meat and rice are fully cooked (internal temperature should register 160°F as measured by a digital meat thermometer), remove from the oven and cool before serving with freshly chopped parsley, if desired.