Preheat your oven to 375°F (190°C).
Grease a large baking dish with a bit of butter or non-stick spray.
Place the sliced Vidalia onions in an even layer at the bottom of the baking dish. Distribute pats of butter across the top of the onions, followed by sprinkling salt, garlic powder, and oregano evenly over them.
Sprinkle the Italian blend cheese generously over the seasoned onions, then add another layer of cheddar cheese on top.
Cover the baking dish with aluminum foil and place it in the preheated oven. Bake for about 30 minutes until the onions are tender and fragrant before removing the foil to allow for browning.
Once baked, remove from the oven and let it cool slightly before serving.