Peel the onions and cut them into thick slices, approximately 1/2 inch (1.27 cm) thick. Separate the slices into rings and place them in a single layer in a baking dish.
In a small bowl, mix together the shredded cheddar cheese, mayonnaise, grated Parmesan cheese, garlic powder, paprika, salt, and pepper. This creates a flavorful cheesy topping.
Spoon the cheese mixture evenly over the onion slices, covering them with the mixture.
Cover the baking dish with aluminum foil and bake in the preheated oven for about 25 minutes.
Remove the foil and bake for an additional 15-20 minutes or until the onions are tender, and the top is golden and bubbling.
If desired, garnish with chopped fresh parsley for added color and flavor.
Serve hot as a savory side dish that complements various main courses.
