At least 12 hours before grilling your beer brats, marinate the brats. Add the bratwurst sausages, sliced onion, and beer to a large airtight container or resealable bag. Shake to combine. Marinate in the refrigerator for at least 12 hours, or for up to 24 hours.
Prepare your grill for zone grilling, creating a zone of direct high heat & a zone of direct low heat. If using a gas grill, begin preheating at least 15 minutes prior to grilling. If using charcoal, light the charcoal at least 30 minutes prior to grilling.
Transfer the brats (& onions, & beer) to a medium cast-iron skillet or a heat-safe Dutch oven. Transfer the cast iron/Dutch oven directly to the grill grates over the low heat zone. Close the grill. Bring the brats up to a boil, then boil for 10-12 minutes or until they reach about 145 degrees F. Between the time it takes to bring the brats to a boil & boiling the brats, this should take about 20-25 minutes total. Be patient & keep the grill lid closed as much as possible so you don’t release unnecessary heat from the grill – it’ll only make things take longer!
Once boiled, use grill tongs to carefully transfer the beer boiled brats directly to the grill grates. Grill 2-3 minutes per side, until charred as desired & the internal temperature reaches 160 degrees F. Transfer to a plate to rest & cool slightly before serving.