Spinach, Caramelized Onion, and Feta Quiche

To reduce the fat content, we used 2 egg yolks and 5 egg whites in this quiche recipe. To make a creamy filling, you need some egg yolks. The remaining yolks can be stored in the refrigerator for up to 3 days in an airtight container (add a tablespoon of water to prevent a film from forming on the yolks).
Preheat the oven to 350 degrees.Heat the olive oil in a large nonstick skillet over medium-high heat. Add the onion, sugar and salt; cook for 30 minutes or until golden brown, stirring occasionally. Add the potatoes and cook for 5 minutes or until they are lightly browned. Remove from heat.Roll out the dough and divide it into strips. On a flat surface, wrap one strip of dough around yourself in a spiral pattern. Add a second strip of dough to the end of the first strip, pinching the ends together to seal; continue to roll up the dough. Repeat the procedure with the remaining dough strips. Cover and let the dough sit for 10 minutes. Roll out the dough into a 12-inch circle and place on a 10-inch deep pie dish greased with cooking spray.Put the potato mixture on the bottom of the prepared cake, and sprinkle the spinach on top. Combine the milk, egg whites, eggs, and cheese; pour the milk mixture over the spinach. Bake at 350 ° C for 1 hour or until set, covering the cake with foil after 50 minutes. Let stand for 10 minutes before serving.