In a lightly oiled heavy bottom pan or rondeau, over medium high heat, sear bratwursts on all sides. Remove bratwurst and set aside.
Melt butter in pan. Add onions and cook until caramelized. Add garlic and cook until fragrant.
Return bratwurst to pan, add sauerkraut, beer and brown sugar. Stir to combine, bring to a simmer and season with salt and pepper to taste. Cover and cook on low until bratwurst are fully cooked. Hold hot for service.
For each order, line pretzel bun with 2 slices Swiss cheese. Place 1/3 cup of sauerkraut mix over cheese. Top with bratwurst and drizzle with 1 tablespoon mustard. Serve hot.
Sign up for 'In Taste,' the McCormick for Chefs newsletter, to receive exclusive offers and rebates, news and insights on the latest industry trends, creative flavor solutions, menu inspiration and more!