Remove the skin from the chicken breasts. Using kitchen scissors, cut each breast in half lengthways but without cutting the whole way through. Open each breast out flat. Brush with the oil and season well with salt and pepper. Heat a griddle pan until very hot. Add the chicken breasts and cook for 3 minutes on each side or until cooked through and grill-marked.
Meanwhile, to make the salsa, mix together the onion, tomatoes, cucumber, red chilli, coriander and lime juice. Season well with salt and pepper.
Serve the chicken hot with the spicy salsa spooned over and around.