Place chicken in a bowl with the lime juice, salt and cumin. Cover and marinate in the fridge 1 hour.
Preheat the barbecue to medium high and oil the grate.
Cook the chicken 3 minutes a side or until cooked through. Cook the onions 4 minutes a side until crisping. Cook the capsicums, skin side down, 10 minutes or until blackened.
Cut the chicken into strips and cut the onion rounds in half. Peel the capsicums and cut into strips. Put the chicken and vegetables in a bowl and squeeze over the remaining lime. Sprinkle with salt to taste. Divide between the tortillas and sprinkle with feta before rolling up.