- 3 pounds boiling potatoes
- 4 large onions
- 24 (17- by 12-inch) phyllo sheets, thawed if frozen
- Vegetable oil spray
- Salt and pepper
- Cut potatoes into quarters. Simmer until tender, about 30 minutes, then drain in a colander.
- Roughly chop onions and cook until golden brown.
- Preheat oven to 375°F with racks in upper and lower thirds.
- For the phyllo, you’ll want to use a flat surface like a cutting board that’s big enough for a sheet of phyllo. Lay down a single sheet of phyllo and spray lightly with vegetable oil (or you can use a pastry brush, but that’s a bit more work). Repeat this three times so you have a stack of three lightly oiled sheets. While you are working with each stack, cover the remaining phyllo with a lightly damp towel in order to prevent it from drying out.
- Cut the stacked phyllo crosswise into 3 strips with a sharp knife or pizza wheel.
- Put 2 tablespoons of filling near the corner of each strip and fold corner of phyllo over to enclose filling and form a triangle. Continue folding the strip, maintaining the triangle shape. Place seam side down on baking sheet. Repeat with remaining phyllo and filling.
- Spray both sides of each triangle with oil and bake, turning at least once. Bake until golden and crisp all over, about 25 minutes total. Cool slightly. Serve warm or at room temperature.
3 sheets make 3 knishes. One box (25 sheets) of phyllo makes 24 knishes.