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 and toast cut-side down on the grill for 1-2 minutes until golden.
04 - Warm sauerkraut in a small saucepan over low heat or in microwave-safe dish until heated through.
05 - Place each grilled bratwurst in a toasted bun. Top generously with sauerkraut and 1 tablespoon mustard. Add sliced red onion and parsley if desired.
06 - Serve immediately while hot.