01 - Preheat grill to medium heat (about 350°F / 175°C).
02 - Place beef bratwursts on grill. Cook, turning occasionally, until browned on all sides and heated through, about 12-15 minutes.
03 - Optional: During the last 2 minutes, lightly brush inside of buns with butter or oil and toast cut-side down on grill until golden.
04 - Warm sauerkraut in small pan over low heat or in microwave.
05 - Place each grilled bratwurst in toasted bun. Top generously with sauerkraut and 1 tbsp mustard. Add sliced red onion and parsley if desired.
06 - Serve immediately while hot.