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1
Blend yogurt, soaked cashews, and 60ml water until smooth — soaking softens the cashews so they break down completely into the paste, eliminating any gritty texture in the finished sauce.
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2
Heat oil in heavy-bottomed pan; render onion slices until deep golden, 8 minutes — this depth of caramelisation builds the korma's characteristic sweetness and body, which paler onions cannot provide; remove.
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3
Add ginger and garlic to pan; bloom 90 seconds — raw heat drives off the harsh, acrid volatiles so their sweeter aromatic compounds come forward — then add all ground spices; toast 2 minutes, which opens the fat-soluble flavour compounds in the spices and fixes them into the oil, where they will disperse evenly through the sauce.
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4
Return onions; add chicken pieces and seal briefly on all sides, 3 minutes — this light searing firms the surface proteins, preventing the pieces from breaking apart during the long simmer that follows.
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5
Stir in cashew-yogurt mixture and coconut milk — adding these together moderates the temperature drop, preventing the yogurt from splitting under sudden heat — season with salt and pepper.
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6
Simmer gently 25–30 minutes until chicken is tender and sauce coats the back of a spoon — a vigorous boil would cause the yogurt and coconut milk to separate, producing a greasy, curdled sauce rather than the velvety emulsion the dish requires.
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7
Finish with lemon juice — added off the heat at the very end, it lifts and sharpens the richness of the coconut and cashew without cooking off its volatile brightness — and adjust seasoning.
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8
Serve garnished with fresh coriander and reserved crisp onions.