Melt the butter in a large, nonstick skillet over medium heat. Stir in the onion, and cook until very tender, about 10 minutes. Stir in as much cabbage as will fit in the pan, and cook until the cabbage is very tender, about 15 minutes. Stir in the remaining cabbage as the mixture cooks down.
Meanwhile, fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the egg noodles, and return to a boil. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through, but is still firm to the bite, about 5 minutes. Drain well in a colander set in the sink.
Stir the kielbasa into the cabbage mixture and cook a few minutes to warm. Fold in the egg noodles, then gently fold in the pierogis. Cover, and cook until the pierogis are hot in the center, about 10 minutes more. Season to taste with salt and pepper before serving.
