Slice the onion thinly, chop the parsley, and drain the sauerkraut well in a colander. Measure the mustard, honey, and red pepper flakes so they are ready to go. Having everything at hand keeps the cooking stress-free.
Place a large skillet over medium heat and add the butter. Let it melt fully and start to shimmer. This gives the onions a rich base to cook in.
Add the sliced onions to the melted butter. Cook them for two to three minutes, stirring often so they soften without burning. Once they turn slightly golden at the edges, they are ready for the sausage.
While the onions cook, cut the kielbasa into two-inch pieces. Push the onions aside and set the sausage directly on the hot pan surface. Turn the pieces every minute for four to five minutes, until they are browned all over.
Add the drained sauerkraut, Dijon mustard, honey, and red pepper flakes to the skillet. Toss everything gently so the sauerkraut does not get squished. If it feels dry or sticky, add one to three tablespoons of water, a little at a time.
Keep stirring for two to three minutes, until the sauerkraut is well seasoned and hot. Sprinkle the fresh parsley over the top. Serve it warm, straight from the skillet.