Slice chicken into strips. Cut onion into wedges, capsicum into strips.
Place in a bowl; add soy sauce, kecap manis, black pepper and a good slurp of oil. Add equal quantities of the sauces untill the mixture is a darkish colour; leave to marinate for at least half an hour.
Place finely sliced lettuce, grated cheese, sour cream and diced tomatoes in four separate bowls.
Meanwhile, heat non-stick wok or saucepan (no need to use oil as oil already in the mix). Once hot, add small batches of meat mixture and stir fry. After each batch, place in a bowl.
When the last batch is cooked, add all the mixture for a quick heat through, being careful not to overcook the chicken.
Place tortillas in cling film and heat in microwave for approximately 75 seconds. When heated, wrap in a clean tea towel and place on table. Guests can assemble their own fajita.