- Place potatoes in large pot of cold salted water. Heat water to boiling over medium-high heat; boil potatoes 10 minutes or until just tender, stirring occasionally. Drain potatoes and rinse under cold water. Cut each potato in half.
- Meanwhile, heat large cast iron or other heavy skillet over medium-high heat. Add 1 tablespoon oil and bratwurst to skillet. Cook 5 minutes or until browned, turning frequently. Add 1/2 cup water to skillet and reduce heat to medium-low. Cover and cook 10 to 12 minutes or until internal temperature of bratwurst reaches 160 degrees F. Transfer bratwurst to plate and cover to keep warm.
- Drain any liquid remaining in skillet and return skillet to medium heat. Add remaining 3 tablespoons oil to skillet. Add onion and potatoes and cook 6 minutes, stirring only occasionally (stirring too frequently will prevent potatoes from browning). Stir in apples and cook mixture 7 to 8 minutes longer or until potatoes are golden brown.
- Meanwhile, in small bowl, whisk vinegar, mustard, sugar, salt and 3 tablespoons water until smooth.
- Stir vinegar mixture into skillet. Remove from heat and let stand 2 minutes. Stir in parsley. Return bratwurst to skillet and serve immediately.