Finely chop 4 of the chillies – deseed a couple or all of them first if you don’t like very spicy food. Split the other 2 chillies from stalk to tip on 1 side without opening or removing the seeds. Cut each chicken breast into 7 or 8 bite-sized chunks.
Heat a tablespoon of the oil in a large, fairly deep, non-stick frying pan (or wok) over a high heat. Add the garlic, chopped chillies, chopped tomatoes, cumin, garam masala, turmeric, sugar and salt, then stir-fry for 3–4 minutes until the vegetables soften. Don’t let the garlic or spices burn or they will add a bitter flavour to the sauce.
Next, add the chicken pieces and whole chillies and cook for 3 minutes, turning the chicken regularly. Pour over the 200ml of water, stir in the yoghurt and reduce the heat only slightly – you want the sauce to simmer. Cook for about 8 minutes, stirring occasionally until the chicken is tender and cooked through and the sauce has reduced by about a third. The yoghurt may separate to begin with but will disappear into the sauce.
While the chicken is cooking, heat the remaining tablespoon of oil in a clean pan and stir-fry the onion and pepper over a high heat for 3–4 minutes until lightly browned. Add the quartered tomatoes and fry for 2–3 minutes more, stirring until the vegetables are just tender. Mix the cornflour with the tablespoon of water to form a smooth paste.
When the chicken is cooked, stir in the cornflour mixture and simmer for a few seconds until the sauce thickens, stirring constantly. Remove from the heat, add the hot stir-fried vegetables and toss together lightly. Serve immediately. And just in case you were wondering – don’t eat the whole chillies!