- Heat olive oil in a large, thick-bottomed deep pot (I used a 5-quart Dutch oven) on medium high heat. Add the chopped onions and cook until translucent and starting to color, about 10 minutes.
- If at any point during the cooking of the onions they start to dry out, add a tablespoon of water to the pot. Stir in the brown sugar, balsamic, salt, and pepper. Lower the heat and cook gently, uncovered, for about 25 to 30 minutes, stirring occasionally, until the onions are nicely caramelized. Then set aside. (You can caramelize the onions up to 2 days ahead and chill until you make the dip.) Let cool a bit before adding to the other ingredients.
- In a medium bowl, mix together the sour cream and mayonnaise (a rubber spatula works well for this). Stir in the caramelized onions. Sprinkle with garlic powder and stir to incorporate. Stir in the Worcestershire sauce.
- Chill until ready to serve.