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

Finely dice the peeled raw onion. Heat a large skillet over medium-high heat; add remaining oil and heat until quite hot. Add diced onion and cook, stirring occasionally, until edges begin to brown, about 5 minutes. Turn to medium-low and cook until onion is golden brown, about 18 minutes. 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.

Puree roasted onions in a food processor. Add mayonnaise, sour cream, vinegar, and salt; pulse until smooth. With the motor running, drizzle in 1/4 cup of the flavored reserved oil. Transfer to a serving bowl. Stir in scallions, pepper, and hot sauce to taste. Refrigerate at least 3 hours. Scatter the fried onions over the top just before serving with potato chips.