In a large pot, combine the beer, sliced onion, salt, and black pepper. Bring the mixture to a boil over high heat.
Add the bratwursts to the pot. Reduce the heat to low and simmer for 10-12 minutes, or until the bratwursts are cooked through.
Remove the bratwursts from the pot and keep warm. Continue simmering the onions for 10-12 minutes, until tender, while the bratwursts are grilling.
Lightly brush the grill grates with olive oil to prevent sticking.
Grill the bratwursts over direct heat for 5-8 minutes, turning every 2 minutes, until browned and lightly charred on all sides.
Place each bratwurst in a bun and top with the cooked onions. Serve with sauerkraut, ketchup, and mustard, if desired.
Notes
Refrigerate leftover bratwursts and onions in an airtight container for up to 4 days. Reheat on the stovetop, grill, or in the microwave until warmed through before serving.You can also pan fry the bratwurst in a large cast iron skillet instead of grilling them to brown them as well.