Make the filling for the Stuffed Cabbage Leaves: Place the bread cubes in a small mixing bowl, add the milk and leave the bread to soak until it is completely saturated, approximate 5 minutes. Squeeze out the excess milk with your hands; discard the milk and add the soaked bread to a large mixing bowl with the meat, rice, parsley, sage, garlic, sugar, salt, and pepper. Mix well, cover with plastic wrap, and refrigerate while you prepare the cabbage.
Steam the cabbage for 5-10 minutes, until the outermost leaves are pliable. After removing several softened leaves, you will likely need to continue steaming the inner leaves. Peel off at least 12 leaves total. Holding each leaf rib-side up, pare the thick part of the rib to flatten it.
Place a small amount of filling in the center of each cabbage leaf. Fold the sides of the leaf over the filling, overlapping them slightly. Starting at the stem end, roll up the leaf into a compact bundle. Continue process until all leaves and meat mixture have been used.
Preheat the oven to 400°F.
Make the sauce: Heat a large Dutch Oven over medium-high heat. Add olive oil, onion, and garlic; saute until the vegetables are translucent. Add tomato paste and paprika; saute for 3 minutes. Stir in chicken stock, tomatoes, balsamic vinegar, sugar, and sage. Season to taste with salt and pepper. Simmer for 5 minutes.
Remove 1 cup of the sauce to a measuring cup. Arrange the stuffed cabbage leaves neatly in a single layer over the remaining sauce in the Dutch Oven. If you had more filling than cabbage leaves to fill, use the rest to make meatballs, and tuck them in around the rolls. Pour the 1 cup of reserved sauce over the rolls. Transfer the Dutch Oven to the preheated oven and bake until the bundles are cooked through and firm to the touch, 25 to 30 minutes.
Use a large spoon to transfer the rolls to a platter or storage container. With an immersion blender, puree the sauce together with the sour cream. Serve the cabbage rolls with mashed potatoes or rice to absorb the luscious sauce.