Heat a saucepan over medium-high heat. When the pan is hot, add the olive and heat. When the oil is hot, add pancetta and saute, and caramelize for about 10 to 15 minutes. Stir onion into mixture, toss and saute for another 10 minutes, until the onion begins to brown. Add garlic, thyme, and cider vinegar, and remove from heat. Allow mixture to cool for about a 1/2 an hour.
Place cooled mixture into bowl, add mayonnaise and sour cream. Season with black pepper, to taste. Serve over iceberg wedges.
Recipe courtesy of Michael Chiarello