Cooking the bratwurst in beer before grilling tenderizes the casings and ups the flavor value.
In a medium-size saucepan, combine beer and onion. Bring to a gentle simmer and add bratwurst. Simmer very gently for 10 to 12 minutes (this can be done right on the grill grate if you heat your grill before simmering brats).
Prepare outdoor grill with medium-hot coals or heat gas grill to medium-high. Grill bratwurst for a total of 12 minutes, turning occasionally with tongs so as not to pierce the links. The bratwurst should register 160°F on an instant-read thermometer.
While brats are grilling, slice rolls and spread with mustard. Heat sauerkraut in microwave for 30 seconds, and spread about 1/4 cup warm sauerkraut per roll. Add a brat to each roll and serve with German potato salad, if desired.