Halve, peel, and thinly slice 2 ½ pounds yellow onions from root to tip (about 8 cups). Heat ¼ cup olive oil in a large, high-sided skillet or Dutch oven over medium heat until shimmering. Add the onions, ½ teaspoon kosher salt, and ¼ teaspoon black pepper. Stir well to combine. Cover and cook undisturbed for 5 minutes. Stir, cover again, and cook for 5 minutes.

Uncover and continue to cook, stirring occasionally at first and then more frequently once the onions start to brown, until the onions are a rich caramel brown color, 30 to 40 minutes more. Reduce the heat as needed if the bottom of the pan is getting too dark. Make sure to scrape the bottom of the pot when stirring so any browned bits get mixed back into the onions and do not burn.

Remove the pan from the heat. Add ⅓ cup water, scraping the bottom of the pan until almost all of the water has been absorbed. Add 1 tablespoon sherry vinegar or red wine vinegar, 1 tablespoon fish sauce or soy sauce, 1 tablespoon Worcestershire sauce, 1 teaspoon garlic powder, and ½ teaspoon granulated sugar. Stir to combine. Let cool for 10 minutes.

Transfer the mixture to a large bowl. Add 1 (16-ounce) container sour cream and stir to combine. Cover and refrigerate for at least 2 hours or preferably overnight. Taste and season with more kosher salt and black pepper as needed. Finely chop 1 large bunch fresh chives until you have about 2 tablespoons and sprinkle over the dip. Serve with ridged potato chips or crudité.