Heat a large skillet over medium heat. Slice the kielbasa into one-inch pieces. Add to the skillet and cook until brown on both sides, 3-5 minutes. Remove from the skillet.
Melt butter in the skillet and add the sliced onions, stirring to coat in butter. Saute for two minutes and add in gnocchi. Season to taste with pepper. Continue to cook until onions have softened and gnocchi has started to brown. Add kielbasa back to the pan. Heat through. Serve with sour cream and dijon mustard on the side.