Preheat the Oven: Preheat your oven to 400°F (200°C). Spray an 8x8-inch baking dish lightly with non-stick cooking spray and set aside.

Sauté the Base: Place a large skillet over medium heat and melt the butter. Add the diced white, red, and sweet onions. Sauté for 5 minutes, stirring occasionally, until they begin to soften.

Caramelize: Add the chopped green onions and brown sugar to the skillet. Cook for an additional 5 minutes, stirring every couple of minutes to prevent burning and encourage even, sticky caramelization.

Build Flavor: Pour in the white wine vinegar and Worcestershire sauce. Continue to cook for another 5 minutes, or until all the onions are very tender, sticky, and have turned a rich, golden-caramel color.

Mix the Cream Cheese: Remove the skillet from the heat. Add the softened cream cheese directly to the hot onions, stirring continuously until the cream cheese is fully melted and incorporated.

Assemble the Dip: Transfer the cream cheese and onion mixture to your prepared 8x8-inch baking dish. Add the sour cream and mayonnaise to the dish, stirring well until the entire mixture is smooth and evenly combined.

Bake: Top the dip evenly with the shredded Parmesan cheese. Place in the preheated oven and bake for 20-22 minutes, or until the dip is hot, bubbling around the edges, and golden brown on top.

Serve: Remove from the oven and let it cool for just a couple of minutes before serving hot with your choice of bread, crackers, or vegetables. Enjoy!