Preheat a large skillet to medium heat. Add the butter or oil. (If using a good nonstick skillet, no butter is needed.) Slice the onion and apples. Then cut the kielbasa into 1/2 inch rounds. Remove the rosemary leaves from the stems and roughly chop.
Once the skillet is hot, sear the kielbasa pieces for 2-4 minutes, until brown on both sides. Remove from the skillet and set aside.
Next add the onions to the skillet. Sear for 2 minutes to soften, then add the apples and rosemary. Allow the apples and onions the cook until soft and brown, but not mushy. Stir gently as to not smash the apples, about 5-7 minutes. Salt and pepper liberally.
Add the sausage back to the skillet, along with the Musselman's Apple Butter, apple cider vinegar, and 1/2 cup water. Simmer and stir to thicken the sauce for 2-3 minutes. Taste, then salt and pepper as needed. Serve warm.