Mix together the whole grain mustard and jalapeño jelly in a small bowl until well blended. This will be the sandwich spread.

Heat a cast iron skillet over medium heat and brown the kielbasa slices until they are slightly darkened around the edges. Remove them from the skillet and set aside.

Preheat your oven’s broiler.

In the same skillet, add the thinly sliced onions and cook until they begin to brown and soften, stirring occasionally.

Add the drained sauerkraut to the skillet with the onions and cook together for about 8 minutes, or until most of the liquid has evaporated.

Slice the rolls and optionally scoop out some of the inner bread to create more space for the filling.

Spread a generous amount of the mustard and jalapeño jelly mixture on the bottom half of each roll.

Layer on the browned kielbasa slices, followed by the onion and sauerkraut mixture, and top with slices of Gruyere cheese.

Place the open sandwiches under the broiler along with the top halves of the rolls, cut side up. Broil just until the cheese is melted and the edges of the rolls are lightly crisped.

Remove from the oven and spread a little more of the mustard-jelly spread on the top halves. Close the sandwiches and serve hot.