Put all dough ingredients in food processor, pulse a few times,
and then continue to process until the dough forms a ball.
Put a tablespoon of oil on your hands, coat the ball of
dough with the oil, and wrap the dough in plastic.
Refrigerate the wrapped dough for 1 hour.
Meanwhile, preheat the oven to 350° F.
Bake the potatoes in a microwave or conventionaloven,
or boil in water. Peel and mash well.
Fry onions in oil until golden. Mix onions,
Divide dough into 3 parts, and roll into 10x6 inch rectangles. One inch from the edge, along the longer edge of each piece of dough, place 1/3 of the filling in a narrow, high row. Spread the filling evenly, and then, beginning with the edge closer to the filling, roll the dough around the filling, finally sealing the dough along the long edges. Place the long knishes on a large greased cookie sheet, and bake for about 40 minutes. Slice each of the knishes into 8 pieces before serving. Alternatively, the dough can be separated into 24 small pieces, and each piece can be individually filled and baked.
Any uneaten knishes should be refrigerated or frozen and reheated. Be sure to wrap them well so they don't absorb other flavors from the freezer.