Do-Ahead Sheboygan "Brats"

Prepare grill. Grill bratwursts on an oiled rack set 5 to 6 inches over glowing coals, turning occasionally, until cooked through and dark golden brown, about 20 minutes. While bratwursts are cooking, in a 3-quart saucepan heat beer, butter, and onions, stirring occasionally, until warm. Keep marinade warm over low heat. Transfer grilled bratwursts to pan and let stand in marinade until ready to serve 15 to 20 minutes. Remove bratwursts from marinade with tongs. Put a pair of "brats," cut in half lengthwise and stacked on each roll. With a slotted spoon put onions from marinade on brats. Apply your chosen accompaniments to brats.