1) Preheat the oven to 180C/Gas 4.
2) In a roasting pan, rub the 2 onions in their skins with the 2 tbsp oil. Bake until squishy soft, turning them once, about 45 minutes. Set aside to cool completely. Peel the onions and set aside.
3) Meanwhile, finely dice the remaining onion. Preheat a large, heavy-bottomed skillet over medium-high heat. Add the remaining 240ml 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 60ml of the onion oil and let it cool completely. (Reserve the remaining oil for dressings or marinades.)
4) 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 60ml 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 spring onions. Refrigerate until very thick, about 3 hours or overnight.
5) When ready to serve top with the fried onions. Serve with crisps.
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