My favorite Vareniki (or varenyky) is potato/cheese, with sauteed onions and, of course, sour cream. My mother-in-law used to get fresh "farmer's cheese" (rather like a dry cottage cheese) from a local farm. If that wasn't available, she used cream cheese. I've never had vareniki as good as hers -- or course she learned from her mother in Ukraine and was amazing to watch making them. She also made them with a sauerkraut filling and for Christmas there were a few with a poppy seed filling to float one in the borscht. They can also be made with fruit filling although she never did that. Traditionally, the 12 courses served Christmas Eve are meatless, although we didn't strictly stick to that. (The recipe for her walnut torte is a family secret. There is no flour in it but it takes a dozen eggs!)