Heat oil in a large skillet over medium heat. Add the kielbasa and cook until golden brown, about 5 minutes per side. Remove from the skillet. You can also grill the kielbasa if you would prefer.
In the same skillet add an additional tablespoon of oil and the onions. Season with salt and pepper. Cook over medium-low heat stirring occasionally for about 30 minutes or until the onion is golden and soft.
Place a sausage onto a poppyseed bun, add mustard on top of the sausage and top with some of the cooked onions and 2 peppers (if using).