How to Make It
Preheat oven to 350°F. Toss cabbage, onion, oil, salt, and pepper on a rimmed baking sheet until well combined. Spread in an even layer and top with bratwurst. Bake until cabbage is tender and bratwurst is cooked through, about 30 minutes.
Spread 1 tablespoon mustard on cut sides of each bun; nestle in bratwurst and apple slices. Add vinegar to cabbage mixture on baking sheet; toss until cabbage turns bright purple. Spoon about ¼ cup cabbage over each bratwurst and top with parsley.