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, about 12-15 minutes.
03 - Lightly brush the inside of the buns with butter or oil and toast them cut-side down on the grill until golden, about 2 minutes.
04 - Warm the sauerkraut in a small saucepan over low heat or in the microwave until heated through.
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 for best results.