Cube the zucchini (sliced lengthwise into ⅛- to ¼-inch widths, each slice into narrow strips, the mass diced crosswise) and put it to disgorge in salted layers in a bowl for 2 hours. Pour into a sieve, rinse under running water, and pat dry in a towel.
Stew the onion gently in olive oil for about ½ hour without letting it color.
Form a paste of the Parmesan, the garlic purée, and the egg, stir in the zucchini and the onion, add pepper to taste. Roll out half of the pastry, trim to an approximate circle, and place on a lightly oiled baking sheet. Spread the filling evenly over the surface to within about ¾ inch of the border. Roll out the other half of the pastry—slightly wider than the first, fold it, cut a tiny “V” from the center to form a vent permitting steam to escape, moisten the edges of the bottom round of pastry and unfold and seal the top into place, pressing all the way around with your fingers. Roll the sealed edges upward and inward on themselves, forming a slightly thicker ribbon of dough surrounding the filled center, and press firmly all the way around with the back of a fork, forming a striated pattern like the rays of symbolic sun images (or press all the way around with the floured edge of your thumb). Brush (or smear) the surface with olive oil and bake at about 375° for from 45 to 50 minutes or until the pastry is richly golden, turning the heat up or down, if necessary, during the last 10 minutes or so to arrive at the correct coloration.