First, make the filling for the Stuffed Cabbage Leaves: Put the bread cubes in a small mixing bowl, add the milk, and leave the bread to soak until it is completely saturated. With your hands, squeeze out the excess milk. Discard the milk and add the soaked bread to a large mixing bowl with the meat, parsley, sage, garlic, sugar, salt, and pepper. Mix well, cover with plastic wrap, and refrigerate.
Bring a pot of water to a boil and prepare a large bowl full of ice water. Add the cabbage leaves to the pot and blanch them in the boiling water just until they are slightly wilted, about 1 minute; drain them and immediately transfer the leaves to the ice water. When the leaves are cold, remove them from the water and pat them dry.
With a small, sharp knife, cut the thick part of the stem from each cabbage leaf. Place a heaping 1/2 cup of the filling in the center of each cabbage leaf. Fold the sides of the leaf over the filling, overlapping them slightly, and then, starting at the stem end, roll up the leaf into a compact bundle. Set aside.
Preheat the oven to 400 degrees F.
Meanwhile, make the sauce: Heat a large heatproof saucepan over medium-high heat. Add the olive oil, onion, and garlic and saute until the vegetables are translucent. Add the tomato paste and paprika and saute briefly. Then, stir in the chicken stock, tomato sauce, balsamic vinegar, sugar, and sage. Season, to taste, with salt and pepper. Simmer for 5 minutes.
Arrange the stuffed cabbage leaves neatly in a single layer in the saucepan. Bring the sauce back to a boil. Transfer the pan to the preheated oven and bake until the bundles are cooked through and firm to the touch, 25 to 30 minutes.
To serve, use a large spoon to transfer the rolls to a platter or individual heated plates. Spoon the sauce over and around them.