Preheat oven to 350°.
Heat olive oil in a large nonstick skillet over medium heat. Add onion, sugar, and salt; cook for 30 minutes or until golden brown, stirring occasionally. Stir in potatoes, and cook for 5 minutes or until lightly browned. Remove from heat.
Unroll 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 procedure with the remaining dough strips. Cover and let dough rest for 10 minutes. Roll dough into a 12-inch circle, and fit into a 10-inch deep-dish pie plate coated with cooking spray.
Spread potato mixture in bottom of prepared crust, and top with spinach. Combine milk, egg whites, eggs, and cheese; pour the milk mixture over the spinach. Bake at 350° for 1 hour or until set, shielding crust with foil after 50 minutes. Let stand 10 minutes before serving.