1. Preheat your oven to 350°F (175°C).
2. Coat a 9×13-inch baking dish with non-stick cooking spray.
3. Clean and peel the Vidalia sweet onions. Slice them into rounds approximately ¼ inch thick and separate the rounds into individual onion rings.
4. Spread the separated onion rings evenly across the base of the prepared baking dish.
5. Season the onion rings with salt, garlic powder, and oregano.
6. Cut the butter into roughly eight even portions or pats. Distribute these butter pats evenly across the seasoned onion rings in the baking dish.
7. Sprinkle the Italian blend cheese and cheddar cheese evenly over the onion rings. Finish with a final topping of shredded Parmesan cheese.
8. Place the baking dish into the preheated oven. Bake for 35-40 minutes until the cheese is browned and bubbling.
9. For optimal taste, serve the dish promptly while it remains warm.