For the curry dip:
1) Drain the yoghurt in a coffee filter or cheese cloth-lined sieve, set over a bowl, until thickened to about 240ml, about 2 hrs.
2) Heat the oil in a small skillet over medium heat. Add the garlic and cook, stirring, until fragrant, about 2 minutes. Add the curry and cook, stirring, until fragrant about 30 seconds more. Set aside to cool.
3) In a bowl, combine the drained yoghurt, curry mixture, mayonnaise, chutney, spring onion, hot sauce, lime juice, and coriander. Season with salt and pepper, to taste.
For the "really" onion dip:
1) Preheat the oven to 210C/Gas 7.
2) In a roasting pan, rub the 2 onions in their skins with the 2 tablespoons of 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 onion. Refrigerate until very thick, about 3 hours or overnight.
5) When ready to serve top with the fried onions.