3 Michigan Fuji apples, cored and cut into 1-inch pieces
1/4 cup apple cider vinegar
3 tablespoons whole grain Dijon mustard
4 teaspoons sugar
1-1/2 teaspoons kosher salt plus more for potato cooking water
3 tablespoons chopped fresh parsley
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.