01 - Preheat grill to medium heat, approximately 350°F.
02 - Place beef bratwursts on the grill. Cook, turning occasionally, until browned on all sides and heated through, approximately 12-15 minutes.
03 - Lightly brush the inside of buns with butter or oil. Toast cut-side down on the grill for 1-2 minutes until golden.
04 - Heat sauerkraut in a small saucepan over low heat or in microwave until warmed through.
05 - Place each grilled bratwurst in a toasted bun. Top generously with sauerkraut and 1 tablespoon of mustard. Add sliced red onion and parsley if desired.
06 - Serve immediately while hot.