Slice the chicken breasts in half horizontally, then cut them into thin strips. Put them in a bowl, add the remaining ingredients and rub into the chicken with your hands.
Heat the oil for the pepper mix in a large frying pan and fry the onion wedges for 6 mins or until softened. Turn the heat up high so the wedges char slightly at the edges, season well and, using a slotted spoon, lift them onto a baking tray and keep warm.
Add the peppers in batches, cook them the same way, then transfer to the baking tray with the onions. Cook the tomatoes in the same way and add them to the peppers.
Heat a griddle pan or use the same frying pan and cook the chicken in batches over a high heat – allow them to catch a little on the edges but don’t overcook them. Add them to the baking tray to keep warm.
Heat the tortillas on the griddle, then wrap in foil and keep warm in the oven, or heat in the microwave following pack instructions.