I tried this at a bridal shower and just had to have the recipe!

Preheat oven: Preheat oven to 350°F (175°C). Grease a 9×13-inch baking dish.
Prepare onions: Peel and slice onions into 1/4-inch thick rings. Arrange in prepared baking dish.
Make cheese sauce: Combine mayonnaise, sour cream, cheddar cheese, mozzarella cheese, garlic powder, onion powder, and black pepper in a medium bowl. Stir until well combined.
Assemble dish: Pour cheese mixture over onion rings in baking dish.
Add topping: Mix crushed Ritz crackers with melted butter. Sprinkle over onion and cheese mixture.
Bake: Bake in preheated oven for 30-35 minutes, or until onions are tender and cheese is bubbly and golden brown.
Garnish and serve: If desired, garnish with chopped parsley. Serve hot.