Preheat skillet over medium-high heat. Form bratwurst into 4 patties and season with salt and pepper.
Sear patties for 4–5 minutes per side until browned and cooked through. Remove and set aside.
In same skillet, cook sliced onions until caramelized.
Pour in Oktoberfest beer, simmer until slightly reduced.
Return patties to skillet, braise in beer mixture for 5 minutes.
Mix Bavarian mustard and mayonnaise to make sauce.
Toast buns with butter until golden.
Top patties with Swiss cheese, cover until melted.
Assemble burgers with sauce, patty, onions, and pickles.
