01 - In a medium saucepan, combine the beer, apple cider vinegar, sugar, salt, black pepper, and mustard seeds. Bring to a simmer over medium heat.
02 - Add the sliced onions to the saucepan and cook for about 10 minutes, stirring occasionally, until softened and slightly translucent.
03 - Grill or pan-fry the sausages according to package instructions until fully cooked.
04 - Serve the sausages on plates topped with the warm beer-pickled onions.