Instructions: Put the flour, baking powder, and salt in the bowl of a heavy-duty mixer equipped with a dough hook. Add 1 tablespoon of the oil, the eggs, and 2 tablespoons of water. Knead for 3 to 4 minutes, add the remaining oil and as much water as needed to make a smooth dough. Or mix the ingredients in a bowl with a wooden spoon and knead the dough on a floured surface for 6 minutes.

Put the dough in an oiled bowl, turn it to oil it all over, and cover it with plastic wrap. Let it stand for 1 hour.

Preheat the oven to 350 degrees.

Heat the fat or oil in a medium skillet and saute the onions until they are just tender. Mix them thoroughly into the mashed potatoes and add salt and pepper to taste.

Divide the dough in thirds. On a floured surface, roll one piece into a thin rectangle about 10 inches long. Place about 2/3 cup of the potato filling along the long end, about 1 inch from the edge. Roll up dough like a jelly roll and pinch the edges closed. Repeat twice, with the remaining dough and filling.

Arrange the long rolls on a baking sheet, and bake them for about 40 minutes, until browned. Slice and serve.

(Unbaked or baked knishes can be frozen, and baked or reheated. Do not defrost. Unbaked knishes take about 45 minutes, baked 15 to 20 minutes.)

This recipe yields about 2 dozen 1 1/4-inch slices.

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