Mix together the whole grain mustard and jalapeño jelly in a small bowl until well combined to create the flavorful spread.
In a cast iron skillet over medium heat, brown the kielbasa slices until they are slightly darkened around the edges. Remove the kielbasa from the pan and set aside.
Preheat the broiler to high.
In the same skillet, add the sweet onions and cook them until they start to soften and turn golden brown.
Add the sauerkraut to the skillet with the onions and continue to cook for about 8 minutes, stirring occasionally, until most of the juices have evaporated.
Slice the rolls open and, if desired, scoop out a little bit of the inside to make more room for the fillings.
Spread some of the mustard and jalapeño jelly mixture on the bottom half of each roll.
Layer the kielbasa rounds over the spread, then pile on some of the onion and sauerkraut mixture. Top everything with a slice of Gruyere cheese.
Place the assembled sandwiches under the broiler along with the top halves of the rolls, cut side up, to lightly crisp and melt the cheese.
Once the cheese is melted and bubbly, remove the sandwiches from the oven. Spread a little more of the mustard and jalapeño jelly mixture on the inside of the top halves of the rolls and close the sandwiches.