InstructionsPreheat the oven to 350° F.Remove the liquid from the sauerkraut, reserving 1/3 cup.Slice the kielbasa into quarter-inch coins and toss them into a large skillet over medium heat with the olive oil. Cook until browned and remove from the pan.Add the onions, salt, pepper, thyme and fennel to the skillet. Cook for 7 minutes until the onions start to soften and reduce. Add the sauerkraut and cook for an additional 5 minutes. Add the stock, sauerkraut liquid and mustard. Stir and bring to a boil for 3 minutes.Add the sausage back in and stir.Press the pierogis into the liquid. This will allow them to absorb more of the delicious broth! Sprinkle with the cheese and bake for 15 minutes.Serve warm. Enjoy!