Be sure to brown the kielbasa well during the first step. That color adds deep flavor during the rest of the cooking time. Slideshow: America's Best Cured Meats
In a large saucepan or skillet set over medium-high heat, heat butter until foaming, then add kielbasa. Cook, flipping the pieces, until the kielbasa is browned, about 5 minutes. Remove the kielbasa and set aside.
Add the onion to the pan and cook, stirring often, until the onion is soft and slightly browned, about 10 minutes. Add the sauerkraut and cook for a few minutes then return kielbasa to pan. Add the stock and cook until the kielbasa is cooked through, about 30 minutes. Season to taste with salt. Serve.
Can be refrigerated for up to two days. Warm before serving.