Combine all the ingredients for the marinade in a bowl. Mix well and set aside.
Slice the chicken breasts diagonally into 1/2 inch strips. Place in the marinade and gently mix. Leave to marinate for at least 1 hour.
To prepare the cucumber relish. Peel the cucumber and slice in half lengthways, then finely slice into thin strips with a mandolin to keep the strips thin and uniform.
Peel and chop the tomatoes, and finely slice the onion.
Mix all the relish ingredients together in a bowl. Season with a pinch of salt and pepper and set aside.
Preheat the oven to 400ºF.
Once the chicken has marinated, heat a wok or ridged pan over a medium heat until hot. Quickly sear the chicken in olive oil until it takes on a nice rich golden color.
Transfer the chicken strips to a baking tray and bake in the oven for a further 5 minutes.
Remove from the oven and season with salt and ground black pepper. Sprinkle over the spring onions and toasted sesame seeds.
Spoon the relish onto plates along with the chicken strips as shown and serve immediately.