Bake potatoes. Preheat the oven to 450F. If you haven’t already, thoroughly wash the potatoes and pat dry. Rub them generously with olive oil, salt and pepper. Use a fork to poke holes all over them so they don’t explode in the oven. Bake them directly on the oven rack for 45-60 minutes, or until tender.
Make filling. While potatoes are baking, chop the kielbasa and the onion into large dice and the cabbage into rough squares or strips. Heat olive oil over medium-high heat in a skillet or cast iron pan. Add the kielbasa and cook until browning. Add the onion and cook until tender, about 10 minutes. Finally, add the cabbage and cook, stirring often, until it has shrunk. Continue to cook,stirring occasionally, until the cabbage is browning and potatoes are done. Don’t stir too often at this point—you want to leave it alone so it’ll brown. Right before you take the potatoes out, stir in the apple cider vinegar and season with salt and pepper to taste.
Fill potatoes. Cut the potatoes in half lengthwise and scoop out flesh, the leaving skins intact if you can. If you can’t that’s okay. They’ll still taste really good. Add butter, garlic, sour cream, mash the potatoes. Salt and pepper the mash to taste. Add mozzarella and the cabbage/kielbasa mixture and gently fold it into the mashed potatoes. Scoop the filling into the potato skins and sprinkle with the rest of the cheese.
Bake and serve. Bake at 350F for 30 minutes. Sprinkle with parsley, if using, and serve.
