Preheat the oven to 350°F. Spray a 10-inch pie plate with cooking spray.
Heat the olive oil in a large nonstick skillet over medium heat. Add the onion, sugar and salt; cook for 30 minutes or until golden brown, stirring occasionally. Stir in the potatoes, and cook for 5 minutes or until lightly browned. Remove from heat.
Unroll the dough, separating into strips. Working on a flat surface, coil one strip of dough around itself in a spiral pattern. Add second strip of dough to the end of the first strip, pinching ends together to seal; continue coiling the dough. Repeat the procedure with the remaining dough strips. Cover and let the dough rest for 10 minutes. Roll the dough into a 12-inch circle, and fit into a 10-inch deep-dish pie plate coated with cooking spray.
Spread the onion/potato mixture in the bottom of the prepared crust, and top with spinach.
In a medium bowl, combine the milk, egg whites, eggs and cheese; pour the milk mixture over the spinach.
Bake for 1 hour or until set, shielding the crust with foil after 50 minutes. Let it stand for 10 minutes before serving.
Notes
If you prefer a more mellow flavor, substitute Swiss or Gruyere for the feta.