Place the chicken breasts in a dish and sprinkle with the lime juice. Cover and marinate for up to 12 hours.
When ready to cook, preheat the oven to 140°C, 300°F, Gas Mark 1.
Stack the tortillas, wrap in aluminum foil and place in the oven to warm through.
Heat the olive oil in a large frying pan, add the onion, and garlic and sauté for 5 minutes, or until just tender.
Meanwhile, cut the chicken into 6mm/1/4-inch pieces.
Stir in the chicken pieces into the frying pan, season with the cumin, salt, and black pepper and cook for 5 minutes, stirring from time to time minutes, until the chicken is opaque and cooked through.
Add the chopped tomato, coriander and the lemon juice, mix well then remove from the heat and transfer to a warm serving dish.
To serve - Stack 2 tortillas (to prevent tearing) and place the chicken, salsa, lettuce on the tortilla stack. Roll it up and lick your fingers.