In a large bowl, mix together the flour, baking powder and salt. Add the oil and rub the ingredients between your hands until their texture is similar to breadcrumbs. Gradually add enough water to made a soft, elastic dough. Cover with plastic wrap and let rest for 2 to 3 minutes.
To make in a food processor, combine the dry ingredients and pulse to mix, then, with the motor running, add the liquids. (Let the dough rest in the food processor for 2 to 3 minutes).
Place the onions and scallions in a colander and sprinkle generously with salt. Knead with your hands for a few minutes to wilt the onions and scallions, then rinse under very warm running water. Drain well and squeeze with your hands to extract as much liquid as possible.
In a large bowl, combine the onions and scallions, cheese, yogurt, breadcrumbs and pepper to taste. Taste and add salt if necessary–feta is usually quite salty. Stir in the eggs.
Preheat the oven to 400°F Oil the bottom and sides of a 13 x 9-inch baking dish and sprinkle with 2 tablespoons of the sesame seeds.
Divide the dough into 3 pieces. Take 2 of the pieces and cover the other. Briefly knead together the pieces and roll the dough out on a lightly floured surface into a sheet large enough to cover the bottom and sides of the baking dish, with a 1/2 inch overhang.
Fit the dough into the dish and add the filling. Roll out the remaining piece of dough into a rectangle just a little larger than the dish and place it over the filling.
Fold the overhanging bottom crust inward. Pinch the two edges together, turn them inward and press to seal, making a neat cord around the edge of the pan. Flatten the cord with the tines of a fork, to prevent it from sticking out, because it will burn during baking. If you like, with a pastry wheel, score parallel lines on the top of the crust, crossing them to make diamond-shaped patterns on the pie, being careful not to cut through the crust.
Brush with the milk and sprinkle with the remaining 2 tablespoons sesame seeds.
Bake for 1 hour, or until the crust pulls away from the sides of the pan and is golden brown on top. Let cool before cutting. Serve warm or at room temperature.