Slice each hotdog almost in half lengthwise, but not quite through to the other side. Press open. Spread 1/2 tsp mustard evenly on both cut side of each hotdog. Place 1 hotdog, cut-side up, on each of 12 individual sheets of heavy-duty (or double layer of regular) foil. Spoon about 2 tbsp sauerkraut onto each hotdog. Spread evenly. Fold edges of foil to enclose each hotdog. Preheat grill to medium. Place packets on ungreased grill. Close lid. Cook for about 5 minutes, turning once, until heated trough. Remove and discard foil.
Spread 1/2 teaspoon ketchup evenly on top half of each bun. Place 1 hotdog on botton half of each bun. Top each with onion, if desired. Cover each with top half of bun.