Using a sharp knife, make a series of small slashes in the flesh of the chicken breasts and rub in the tandoori paste or powder. Leave to marinate in the refrigerator overnight.
Heat a griddle pan (or ordinary frying pan). Cook the marinated chicken breasts for 8–10 minutes on each side, allowing the authentic tandoori charred colour to appear, or until cooked throroughly.
Mix the red onions, tomatoes and coriander together with the lemon juice, olive oil and seasoning in a small bowl. Serve the salad with the tandoori chicken, accompanied by lemon wedges, griddled if liked.