In a large bowl or in the food processor, blend the pureed potato, flour, and salt together.
Add the water and blend well until the dough will hold together in a cohesive ball. With a sharp knife, divide into 10 equal portions. Turn these out, one at a time, onto a floured surface and roll into circles.
Roll as thin as possible, at most 1/8 inch, so each is approximately 7 to 8 inches across. Prick all over with the tines of a fork.
Lightly oil a griddle or large skillet and heat until it is very hot. One by one, place the rolled circles onto the griddle and cook until the bottoms begin to show light browning. Then turn and cook the other side.
Keep turning until the whole cracker is dry and crisp. Do not allow the flatbread to get more than medium brown. Cool on a rack.
VARIATIONS: Substitute any pureed vegetable for potatoes. Flours other than rye can also be used. Try wheat or barley. If the dough is too sticky to roll, add a little more flour. Letting the dough rest for an hour after mixing will also make it easier to roll.
Flatbread can also be cooked in the oven. Bake on an ungreased baking sheet at 400~F. for 8 to 12 minutes, or until crisp, turning 2 or 3 times.