In a saucepan, bring beer and onion to a simmer. Pierce bratwurst casings with a sharp knife. Place brats in the simmering beer, turn down the heat to just below a simmer, and let brats poach for about 6 minutes.
Over medium coals, grill brats until they have good color, turning so they are evenly browned about 1 minute per turn.
If not serving right away, return the cooked brats to the beer/onion mixture to keep hot.
Split rolls almost all the way through. Place, cut side down, on the grill just to get some grill marks.
Gently warm the sauerkraut over medium-low heat. Don't let it boil, you just want to warm it through.
Spread spicy mustard to taste in each bun. Top with a brat and then some warm sauerkraut. Finish with more mustard, if desired.