1. Preheat oven to 350 degrees F. Coat an 8"x8" baking dish with cooking spray and set aside.
2. In a large pot, bring a small amount of water to a boil. Reduce heat to medium, add spinach, cover, and cook until the spinach is wilted (3-5 minutes). Drain and squeeze the spinach dry. Chop and place in a large bowl.
3. Heat oil in a small skillet over low heat. Saute the leek and garlic until tender (10 minutes). Add the leek mixture to the spinach. Stir in the lemon juice, oregano, and pepper.
4. In another large bowl, beat the egg whites until foamy using a mixer. Fold into the spinach mixture.
5. Pour the mixture into the prepared pan and bake until set (35-40 minutes). Let set 10 minutes before cutting into 2" squares to serve.