Place bacon in a large skillet over medium heat and cook until the bacon is crispy (about 2 minutes). Add onion and cook until soft, 4–5 minutes. Add reserved spiced liquid, sauerkraut, potatoes, and sausage. Stir to combine. Cover and cook until the potatoes are tender (about 15 minutes). Add apple slices, cover, turn off heat, and let the apples warm to the temperature of the rest of the dish. Garnish with parsley, and serve with crusty bread and mustard.