1. To make the dough: Combine the flour, baking powder, and salt in a large bowl. Make a well in the center and add the oil, eggs, and 2 tablespoons water. Gradually mix the wet ingredients with the flour. Add up to 2 tablespoons more water if necessary. Knead the dough until it forms a rough ball.
2. Knead the dough on a lightly floured surface until the dough is smooth, about 6 minutes. Place the dough in a lightly oiled bowl, cover, and refrigerate for 1 hour.
3. Preheat the oven to 350°F.
4. To make the filling: Combine the chicken fat, onions, potatoes, and salt and pepper to taste in a bowl.
5. Remove the dough from the refrigerator and divide it into thirds. On a floured surface, roll out one piece of the dough into a thin rectangle about 10 inches long. Spread about 2/3 cup of the filling along one long end, about 1 inch from the edge. Roll up like a jelly roll and pinch the edges closed. Repeat with the remaining pieces of dough and filling.
6. Arrange the rolls on baking sheets. Bake until browned, about 40 minutes.
7. Slice the knishes and serve.