Prep the Ingredients: Finely chop 4 of the chili peppers – 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.
Build the Sauce Base: 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.
Cook the Chicken: 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. (Note: The yoghurt may look separated at first, but it will smoothly incorporate into the sauce as it cooks.)
Sauté the Vegetables: While the chicken is simmering, 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 crisp-tender. In a small bowl, whisk the cornstarch with the tablespoon of water to form a smooth slurry.
Thicken and Combine: When the chicken is cooked, stir in the cornstarch 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!