Preheat oven to 425 degrees F (220 degrees C).
Combine the artichoke hearts, onion, salad dressing, mayonnaise, sour cream, Swiss cheese, mozzarella cheese, 1/2 cup Parmesan cheese, Romano cheese, garlic powder, salt, and pepper in a large mixing bowl; mix evenly. Gently fold the spinach into the mixture; spread into the bottom of an 8x8 casserole pan. Sprinkle 1/4 cup Parmesan cheese over the top. Cover with aluminum foil.
Bake in preheated oven 15 minutes. Remove the aluminum foil and bake until top begins to brown, about 10 minutes more. Allow to cool 10 to 15 minutes before serving.