You know that dip you loved as a child? You sprinkled the dehydrated onion mix into sour cream, opened a bag of chips, and were in heaven. Here's a modern twist made by sauteing five types of onions—Vidalia, yellow, shallots, leeks, and garlic—very slowly, until the natural sugars caramelize. Each type of onion adds its own flavor. The dip can be made a day ahead of time but is best several hours after preparing.
In a very large skillet, heat 2-½ tablespoons of the oil over medium-low heat. Add all the onions. (If your skillet isn't large enough, divide the onions and oil among two pans.) Cook, stirring every 5 minutes or so, until the onions are wilted; then reduce heat to low and continue cooking until the onions are very soft, turning a pale golden color, about 30 minutes more. Season with salt and pepper to taste.
Add the vinegar and cook another 10 minutes, stirring occasionally. Remove from the heat and let cool a bit. Taste for seasoning and add more salt or pepper as needed.
Transfer the mixture to a food processor and blend slightly. You don't want the onions to be pureed, but look for a chunky consistency. Add the sour cream and remaining tablespoon of oil, and blend again, pulsing the machine a few times until incorporated. Check the seasoning and add salt and pepper as needed.