Quarter squash lengthwise and discard seeds. Peel squash carefully and cut into 3/4-inch pieces. In a large heavy skillet cook squash in oil over moderate heat, stirring occasionally, 2 minutes. Add water and salt to taste and simmer, covered, until squash is just tender, about 10 minutes. Simmer squash, uncovered, until almost all water is evaporated, about 5 minutes. In a food processor puree squash with salt and pepper to taste. Squash puree may be made 1 day ahead and chilled, covered. Preheat oven to 400F. Lightly brush a large baking sheet with some butter and put 1 sheet phyllo on butter. Lightly brush phyllo with some remaining butter and sprinkle with 1 tablespoon Parmesan. Put another sheet of phyllo over cheese, pressing it firmly so that it adheres to bottom layer. Butter, sprinkle with cheese, and layer remaining phyllo in the same manner, ending with a sheet of phyllo. Lightly brush top sheet with remaining butter. Fold in all sides 1/4 inch, pressing to top sheet, and fold up a 1/4-inch border, crimping corners. Spread squash puree evenly on phyllo crust and top with bacon, rosemary, scallion greens, and onion. Bake pizza in middle of oven until crust is golden, about 15 minutes. Serves 4 to 6 as an entree or 8 to 10 as an hors doeuvre. Gourmet December 1994