In a roasting pan, rub the 2 onions in their skins with the 2 tablespoons oil. Bake until squishy soft, turning them once, about 45 minutes. Set aside to cool completely. Peel the onions and set aside.
Meanwhile, finely dice the remaining onion. Preheat a large, heavy-bottomed skillet over medium-high heat. Add the remaining 1 cup oil and heat until hot. Add the diced onion and cook, stirring occasionally, until they just begin to lightly brown, about 5 minutes. Lower the heat to medium-low and continue to cook, stirring occasionally, until golden brown, about 18 minutes more. Strain the onions through a sieve over a bowl. Transfer the onions to a paper towel-lined plate and spread them into a single layer. Reserve 1/4 cup of the onion oil and let it cool completely. (Reserve the remaining oil for dressings or marinades.)
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. Transfer the onion dip to a serving bowl and stir in the scallion. Refrigerate until very thick, about 3 hours or overnight.
When ready to serve top with the fried onions. Serve with chips.
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