3 tablespoons extra-virgin olive oil
2 tablespoons finely chopped fresh rosemary
½ teaspoon dried red pepper flakes
½ teaspoon freshly ground black pepper
1 medium yellow onion, finely chopped
1 medium red bell pepper--halved, seeded and finely chopped
4 garlic cloves, very finely chopped
1½ teaspoons kosher salt
3 tablespoons maple syrup
3 tablespoons seedy Dijon mustard
¾ cup finely chopped Peppadew peppers (about 15 Peppadews)
½ cup apple cider vinegar
¼ cup water
1 tablespoon extra-virgin olive oil
4 kielbasa links (about 1 pound)
1 medium red onion, halved then thinly sliced
¼ teaspoon kosher salt
4 Italian hero or submarine buns
¼ cup melted butter or mayonnaise (optional)
1. Make the relish: In a large skillet set over medium heat, add the 3 tablespoons of olive oil. Once the oil shimmers, after 1 to 2 minutes, add the rosemary, red pepper flakes and black pepper. Stir until the rosemary and black pepper are fragrant, 30 seconds to 1 minute. Add the onion, increase the heat to medium-high and cook, stirring occasionally, until the onion is soft and just starting to brown around the edges, 3 to 5 minutes. Stir in the red bell pepper and cook until soft, about 8 minutes, then add the garlic and the 1½ teaspoons of salt and cook, stirring often, until the garlic is fragrant, about 1 minute. Stir in the maple syrup, mustard, Peppadew peppers, vinegar and water, bring to a simmer and reduce the heat to medium. Cook, stirring occasionally, until the relish is thick and glossy and there isn't any liquid pooling in the bottom of the skillet, 12 to 15 minutes more. Turn off the heat and transfer the relish to a medium bowl.
2. Cook the kielbasa: In a large skillet set over medium-high heat, add the 1 tablespoon of olive oil. Add the kielbasa links and brown on both sides, 4 to 6 minutes total. Transfer the kielbasa to a paper-towel-lined plate.
3. Sprinkle the sliced red onion with the ¼ teaspoon of salt and set aside for 5 minutes. Halve the buns lengthwise and tunnel out a narrow strip down the thickest half (the trench will cradle the sausage). Brush with the melted butter or mayonnaise, if using, then place the rolls cut side up under the broiler until toasted, 1 to 3 minutes (watch the bread closely, as broiler intensities vary).
4. Divide the salted onions among the bottom halves of each toasted bun. Add a kielbasa link to the trench of each bun. On the top half of each bun, spread about ¼ cup of relish. Serve.