Preheat oven to 375ºF. Coat a 9-inch nonstick pie pan with cooking spray.
Coat a large nonstick skillet with cooking spray and set pan over medium-high heat. Add onion and sauté until soft and golden, about 5 minutes.
Transfer onions to a large bowl and add spinach, ricotta, cheddar cheese, American cheese, salad dressing, eggs, mustard, oregano, salt and black pepper. Mix until well blended. Spoon mixture into prepared pan and smooth top with back of a spoon. Sprinkle with grated cheese topping.
Bake until a knife inserted near center comes out clean, about 30 minutes. Let stand 10 minutes before slicing into 6 pieces.