Heat a small skillet over medium heat. Toast the seeds a couple of minutes then stir in sauerkraut, beer or cider, and mustard and simmer 10 minutes.
Cut kielbasa into 4 portions and split the sausage pieces in half across, opening them up. Grill the sausages on the hot griddle until crispy on both sides, about 7 to 8 minutes total. Wipe some of the grease off the griddle and drop heat to low.
Lightly butter 1 side of each slice of bread. Build butter-side-out sandwiches of grilled kielbasa, sauerkraut, and 2 slices of cheese; spread the top of each sandwich with red pepper relish before setting in place.
Grill the sandwiches on the griddle until crispy. Cut and serve.