Heat the olive oil in a pan and sauté the leeks for about 5 minutes, then add the onion and continue sautéing until both are soft and tender (another 3 minutes or so).In a large bowl, beat the eggs well then add the yougurt, feta cheese, dill, salt and pepper and sautéed leeks and onions and mix well.Grease the baking pan. Use half the phyllo sheets on the bottom, laying them across the full surface of the pan, brushing each lightly with olive oil. Spread filling evenly over the phyllo and cover with the remaining sheets, brushing each one with olive oil. Bake for about 1 hour in an oven pre-heated to 180° C(350°F).
Allow to cool before cutting. Can be served warm or at room temperature.