Heat grill to medium-low; lightly oil grates. In a medium saucepan, heat butter over medium. Add onion, and cook, stirring occasionally, until soft, 4 to 5 minutes. Add sauerkraut, sugar, caraway seeds, and 1/2 cup water; season with salt and pepper. Simmer until sauerkraut is heated through, 3 to 5 minutes; set aside.
Grill sausages, turning frequently, until golden brown and warmed through, 10 to 12 minutes.
Grill rolls, cut side down, until warm, 1 to 2 minutes. Spread lightly with butter, if using, and mustard. Halve sausages lengthwise; fill each roll with 2 halves and pickles. Serve with sauerkraut and potato chips.
This recipe calls for easy-to-find precooked brats (look near the other sausages). For fresh ones, increase the grilling time by 5 minutes, until they're cooked through.