Put the onion, ginger, garlic, chopped chilli and coriander stems in a food processor and blitz to a coarse paste.
On a medium-high flame, heat the oil in a large saute pan for which you have a lid, then add the onion mix, turmeric and two-thirds of the curry leaves, and cook, stirring, for three minutes, until fragrant. Add the chicken, a teaspoon of salt and a good grind of pepper, and cook, stirring occasionally, for about eight minutes, until the chicken is no longer pink on the outside. Add a teaspoon of the sugar, the coconut milk and 100ml water, bring to a boil, then cover, turn the heat to medium-low, and leave to cook for about 35 minutes, until the chicken is the chicken is cooked through and tender.
Use tongs to lift out the chicken, turn up the heat to medium-high and cook the sauce for five minutes, until reduced by a third. While it’s reducing, roughly shred the chicken with two forks into bite-sized pieces. Off the heat, stir the chicken back into the sauce with the lime juice and coriander leaves, and keep warm.
Meanwhile, make the crumble. Put the oil and sliced chilli in a medium saute pan on a medium-high heat. Cook for five minutes, or until the chilli starts to soften, then add the mustard seeds and remaining curry leaves, and fry for 30 seconds, until fragrant. Add the oats, peanuts, remaining tablespoon of sugar and a third of a teaspoon of salt, and cook for three minutes, stirring often, until lightly coloured and crisp.
To serve, sprinkle the crumble over the chicken, top with the lime zest and extra coriander, and serve straight from the pan.
