Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
Prepare the Onion Layers: Arrange the onion slices in an even layer in the prepared baking dish, separating the rings slightly.
Make the Creamy Topping: In a medium bowl, mix together the mayonnaise, sour cream, shredded cheddar cheese, shredded mozzarella cheese, garlic powder, onion powder, and black pepper until well combined.
Spread the Mixture: Spread the cheese and cream mixture evenly over the top of the onion slices, making sure to cover all the onions.
Add the Topping: In a small bowl, mix the crushed Ritz crackers with the melted butter. Sprinkle the buttered crackers over the top of the cheese mixture.
Bake: Bake in the preheated oven for 25-30 minutes, or until the cheese is bubbly and the top is golden brown.
Garnish and Serve: Remove the dish from the oven and let it cool slightly. Garnish with chopped fresh parsley if desired, and serve warm as a delicious side dish.