Watch and listen to this recipe from Sara Moulton.
View the full recipe here.

1. Roast
Rub unpeeled onions with a bit of the olive oil; roast until soft, about 45 minutes. Cool and peel.

2. Brown
Finely dice, then cook the remaining onion over medium-high heat, stirring occasionally, until light brown, about 5 minutes. Lower the heat to medium-low and continue to cook, stirring occasionally, until golden brown, about 18 minutes more.

3. Strain
Scrape onion, oil, and juices into a sieve over a bowl. Drain onions and spread on a paper towel-lined plate. Cool the strained oil.

4. Puree
Puree the roasted onions in a food processor. Add the mayonnaise, sour cream, vinegar, and salt, and pulse until smooth. While the motor is running, drizzle in the 1/4 cup of reserved onion oil until incorporated. Season with pepper and hot sauce, to taste.

5. Serve
Refrigerate at least 3 hours. Scatter the fried onions over the top just before serving with potato chips.