Begin by preheating the oven to a temperature of 350°F (175°C). Subsequently, coat a 9x13-inch baking dish with a suitable non-stick cooking spray.

After cleaning and peeling the Vidalia sweet onions, position them sideways and proceed to slice them into rounds that are approximately ¼ inch in thickness. Following this, it is necessary to separate the sliced rounds into individual onion rings.

Next, arrange the separated onion rings uniformly across the base of the previously prepared baking dish. Once arranged, season the onion rings evenly with the salt, garlic powder, and oregano.

The butter should then be cut into roughly eight even portions or pats. Distribute these butter pats evenly across the seasoned onion rings in the baking dish.

Continue by uniformly sprinkling the Italian blend cheese and cheddar cheese over the arranged onion rings. Finish this cheese layering with a final topping of the shredded parmesan cheese.

Once assembled, place the baking dish into the preheated oven. Allow the dish to bake for a duration of 35-40 minutes. The dish should be removed when the cheese has begun to brown and bubble.

For optimal taste, it is advised to serve the dish promptly while it remains warm.