In a large stockpot over medium high heat, add brats, onion beef stock and beer.
Bring the brats to a boil and reduce the heat to low. Simmer for 15 minutes or until the brats are cooked through.
Remove the brats from the beer and keep warm. Continue to cook the onions for an additional 5 minutes if using as a topping.
Transfer the brats to the grill and cook, turning frequently, until browned.
Serve the brats in buns with mustard, sauerkraut and onions, if using.
Notes
Beer brats can be boiled up to 2 days in advance and grilled right before serving. Serve boiled brats in an airtight container in the refrigerator and remove before grilling.