Oven Preheating: Initiate the process by preheating the oven to 350°F (175°C). Concurrently, prepare a 9×13-inch baking dish by coating it evenly with a non-stick cooking spray.

Onion Preparation: Post cleaning and peeling, position the Vidalia onions on their sides. Proceed to slice them into rounds of approximately ¼ inch thickness.

Separation of Onion Rounds: Post slicing, each round should be delicately separated into individual rings.

Arrangement and Seasoning of Onion Rings: Lay the separated onion rings flat within the pre-coated baking dish. Follow this by evenly distributing the salt, garlic powder, and oregano over them.

Butter Application: Divide the unsalted butter into approximately eight equal parts or pats. These should then be evenly placed atop the seasoned onion rings.

Cheese Layering: Commence by evenly sprinkling the Italian blend cheese, followed by the cheddar cheese over the onion rings. Conclude this step by topping it with the finely shredded parmesan cheese.

Baking Process: Transfer the baking dish into the preheated oven. Allow it to bake for a duration of 35 to 40 minutes, or until the cheese on top begins to brown and bubble.

Serving: It is recommended to serve the dish promptly while it is still warm to ensure the best flavor and texture.