Place yeast in large mixing bowl and add mashed potatoes and potato water. Add sugar and salt.
Stir and let stand until thoroughly dissolved, about 5 minutes. Add eggs and mix.
Add about half of the flour, 3 1/2 cups, Beat with spoon until smooth and very elastic. Batter will fall from the spoon in sheets. Beat in cooled shortening.
Add in most of the remaining flour and knead well until dough is easy to handle. Turn dough onto a lightly floured bread board. Cover and let stand for 10 minutes to tighten up then, knead until smooth and elastic.
Put into a large greased bowl, grease top of dough, Cover and store in refrigerator.
Melt shortening in Dutch oven. Add onions and cabbage. Steam them stirring so that they do not over cook.
Brown the ground beef and add to the onion and cabbage mixture. Mix well and season to taste.
Roll refrigerated dough on floured board to about 1/2 inch thickness.
Cut into 5 inch squares and place heaping tablespoon of meat and cabbage mixture in the center of each square.
Bring 4 corners of dough together and pinch cut edges.
Place on greased baking sheet pinched side down. Set in warm place to rise 15-20 minutes