Preheat oven to 350°F (175°C).
In a medium bowl, combine chopped artichoke hearts, finely chopped onion, creamy salad dressing, mayonnaise, and sour cream.
Stir in shredded Swiss, mozzarella, Parmesan, Romano cheeses, and garlic powder.
Add the thawed and drained spinach, mixing well to combine all ingredients evenly.
Season with salt and black pepper to taste.
Transfer the mixture to a greased baking dish and spread evenly.
Sprinkle the remaining 1/4 cup Parmesan cheese evenly over the top.
Bake uncovered for 20-25 minutes, until hot and bubbly and the top is slightly golden.
Remove from oven and let cool for a few minutes before serving with crackers, bread, or fresh veggies.