Over medium heat, melt the butter in a skillet. Add the white, red, and sweet onion and sauté for 5 minutes. Add the green onions and brown sugar and cook for an additional 5 minutes, stirring every couple minutes.
Add the white wine vinegar and worcestershire sauce. Cook for another 5 minutes, or until the onions are all very tender and turning a golden caramel color.
Add the cream cheese to the skillet and stir to combine with the onions.
Transfer the mixture to an 8×8 baking dish and stir in the sour cream and mayonnaise.
Top with parmesan cheese and bake for 20-22 minutes or until golden brown on top.