Heat a skillet over medium heat and add a generous pat of butter. Add the sliced onions with a pinch of salt and cook low and slow, stirring occasionally, until deeply golden and caramelized.
While the onions cook, preheat your grill or grill pan to medium-high heat. Score the kielbasa links with a few shallow diagonal cuts on each side to help them cook evenly and get the great grill marks.
Grill the kielbasa for about 4-5 minutes per side until nicely charred and heated through.
Toast the hoagie, if you wish!
Spread whole grain mustard generously on the toasted rolls. Nestle a grilled kielbasa link inside each roll and top with a heap of caramelized onions. Serve immediately.
