• Preheat oven to 375°F.

MAKE THE FILLING:

• In a large heavy skillet cook the onion in oil over moderately low heat, stirring, until softened.
• Squeeze the spinach to remove as much liquid as possible and stir with salt into the onion. Cook the spinach mixture over moderate heat, stirring occasionally, until any liquid is evaporated, 1 to 2 minutes, and cool slightly.
• In a large bowl whisk the eggs until combined and stir in the spinach mixture, pine nuts, feta, and bread crumbs until combined well. (the filling may be made 1 day ahead and chilled, covered. Bring the filling to room temperature before proceeding.)

MAKE THE TART:

• Stack the phyllo sheets and cover with 2 overlapping sheets of plastic wrap and then a lightly dampened kitchen towel. In a small saucepan melt the butter and then let it cool slightly. Lightly brush a 10½ x 7½ x 1-inch rectangular tart pan with a removable fluted rim with butter.
• On a work surface lightly brush 1 phyllo sheet with butter. Sprinkle 1 tablespoon of Parmesan evenly over the buttered phyllo and repeat the layering with 5 more phyllo sheets, butter, and 5 tablespoons of Parmesan. Arrange the last phyllo sheet on the stack and lightly brush with butter.
• Transfer the phyllo to the tart pan, letting the excess hang over the edge. Spoon the filling onto the phyllo, spreading evenly. Fold the edges of the pastry over the filling, leaving the center uncovered. Lightly brush the top of the phyllo with butter. Sprinkle the remaining Parmesan over the exposed filling and bake the tart in the middle of the oven until golden, 25 to 30 minutes. Serve the tart warm or at room temperature.