Cover the bottom of a skillet with softened butter. Arrange pierogis in the pan in a single layer. Add 1 cup water to the pan. Cover pan and place over medium high heat. Cook covered 8 minutes, remove lid and cook out any liquid in the pot.
Heat a large nonstick skillet over medium high heat. Brown kielbasa and remove from pan. Cover kielbasa with foil to keep warm. Drain fat and add 1 tablespoon vegetable oil and 1 tablespoon butter. Raise heat to high. Add onion to the pan and saute for a few minutes to soften. Add chopped kale in batches as it wilts into the pan. When kale is wilted, add sauerkraut. Season stir-fry with mustard, paprika, salt and pepper. Return kielbasa to pan. Toss to combine and transfer to a serving platter.
Let the pierogis "stick" and brown in the butter as the liquid evaporates. Remove crisp pierogis from heat to a plate.
Stir together the chopped herbs and sour cream and season with salt and pepper. Serve with pierogis.
Recipe courtesy of Rachael Ray