Place brats and onions in a large, deep pan or Dutch oven. Pour beer over top until brats are completely submerged. Bring to a boil.
Reduce heat to low and simmer until the brats are cooked through (about 15-20 minutes).
Remove brats and place on a tray.
Grill brats over medium-high heat until golden brown (about 3-5 minutes per side). Serve immediately, or return to beer mixture until ready to serve.
