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Make the Chilli-ginger garnish (see right), before you start the chicken.
Cut down the length of the green chillies, then remove the seeds by scraping them out with a teaspoon. You may want to wear rubber gloves for this job - fiery seeds can give the fingers a real chilli sting. Cut off the stalk ends, chop the chillies into rough pieces and put them in a blender or food processor. Peel and roughly chop the garlic and add to the chillies. Do the same with the ginger.
Pick the leaves off the coriander and drop them into the blender as you go (reserve the stems for the garnish). Tip in the yogurt and blitz everything to a puree. This can take a few minutes and you may have to stop the blender a few times to scrape all the gunge down from the sides - a rubber spatula will make this job much easier.
Peel, halve and thinly slice the onions. Pull the skin off the chicken, using kitchen paper to help you get a good grip. Heat the oil in a saute pan over a medium heat until hot - when you drop in a spice seed or two it should sizzle. Toss in the fennel and cumin seeds and let them crackle and pop for 10-20 seconds, stirring all the time so they don't burn. They're ready when you get a lovely whiff of warm nuts. If the oil starts to smoke, remove pan from the heat.
Plop in the butter and let it melt, then throw in the sliced onions and turn the heat down a tad. Stir to mix the onions and spice seeds, then cook for about 10 minutes until the onions are meltingly soft and tinged golden brown. Give them a vigorous stir now and then, but don't stir them too often or they won't take on any colour.
Push the onions to one side of the pan, then add the chicken thighs, smooth side down. Increase the heat to between medium and high and fry the chicken until it's golden brown on both sides. This should take about 15-20 minutes and you will need to turn the pieces over halfway (a pair of tongs are good for this). Watch carefully in case the onions start to brown too much - if they do, pile them up on top of the chicken.
Stir in some of the yogurt mixture and cook until it disappears - repeat a dollop at a time. Whizz 150ml/¼ pint cold water in the blender to use the last bits of the mixture and add to the pan with a pinch of salt. Stir until bubbling, scraping up any crispy bits. Reduce the heat to its lowest and cover the pan tightly. Simmer for 30-35 minutes or until the chicken is tender when pierced, stirring and turning halfway and adding a splash of hot water if the sauce is too dry. Serve sprinkled with finely chopped coriander stems and a little chilli garnish (put the rest in a bowl on the table).