Pickled red onion slice red onions. Place in a bowl add vinegar, and sugar salt and pepper mix well and refrigerate for three hours. Next dice up bacon and onion. In a sauté pan render the bacon until crispy add the onion and sautéed next add the sauerkraut cook on medium heat for two to three min add some beer cook down until most of the liquid has evaporated.grill the bratwurst until cooked all the way. To assemble the sandwich slice the bratwurst in half long way, place on the bottom of kaiser roll next spread mustard on top of roll then top the bratwurst with the sauerkraut mix and top off with pickled red onions.