Prepare a smoker or grill with smoke box to cook at 250°F (120°C). Get a steady stream of smoke going before cooking the sausages.
Once you have a steady stream of smoke, place the bratwurst on the smoker or grill and cook, flipping and rotating once, and replenishing the wood as needed, until the sausages reach an internal temperature of 160°F (71°C), about 90 minutes.
Remove the sausages from the smoker and tuck into buns. Top with sauerkraut and mustard, if desired.
Notes
Don't poke too often--Wait for at least 1 hour and 15 minutes before checking the internal temperature of your brats and avoid poking them frequently. Poking them multiple times will cause the juice inside to run out, leaving you with dry sausages.