Preheat the Oven: Preheat your oven to 375°F (190°C).
Prepare the Onions: Peel the Vidalia onions and slice them into thick rings, about 1/4 inch in thickness. Separate the rings and arrange them in a single layer in a greased 9x13-inch baking dish.
Season the Onions: Pour the melted butter evenly over the onion rings. Sprinkle the salt, garlic powder, and oregano over the onions, ensuring even coverage for maximum flavor.
Add the Cheese: Sprinkle the shredded Italian blend cheese evenly over the seasoned onions. Follow with the cheddar cheese, and finally, top with the grated Parmesan cheese.
Bake: Cover the baking dish with aluminum foil and bake in the preheated oven for 30 minutes. Remove the foil and continue baking for an additional 15-20 minutes, or until the cheese is bubbly and golden brown on top.
Serve: Remove the dish from the oven and let it sit for a few minutes before serving. Tennessee Onions are best enjoyed warm as a side dish with grilled meats, casseroles, or even as a topping for burgers.