Cook pasta shells according to directions. Meanwhile, saute onion if you're using it.
When shells are cooked, rinse with cold water to cool. Put mashed potatoes in a mixing bowl, add cheese to taste (I use about a half cup cheese to 2 cups mashed potatoes) and onions or sauerkraut if you're using. Mix well.
Fill each shell with 1-2 tablespoons potato mix and place in a baking pan.
Top each stuffed shell with a small amt of sour cream (1/2 tsp maybe?) spread evenly over top. Bake in 350 oven for 15-20 minutes, just to heat through.
Serve topped with more shredded cheese and/or more sour cream. Goes quite nicely with kielbasi!