Heat some olive oil in a large pan over medium high heat. Add the onion slices, along with a couple pinches of salt. Cook for 10 minutes, stirring occasionally. Stir in the soy sauce and balsamic vinegar. Cook for 2 minutes more, and transfer to a bowl.

Wipe out the pan with a paper towel, and add a little more olive oil, along with the kielbasa and potato slices. Cook over medium-high heat for a couple minutes. Cover with a lid and turn the heat to medium-low. Cook for 20 minutes, stirring occasionally. Let the fat from the kielbasa slices render and flavor the potatoes. If the potatoes or kielbasa start to brown too quickly, add a splash of water and continue to cook. Cook until the kielbasa is browned and the potatoes are tender. Add back the onions and toss together. Season with salt and pepper if necessary. Transfer to a serving platter and garnish with chopped scallions.