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