Heat the oil in a large frying pan over high heat. Add half the chicken and cook, turning occasionally, for 3-4 minutes or until chicken is golden brown and cooked through. Transfer to a bowl. Repeat with remaining chicken. Add the capsicums and onion to the pan and cook, stirring, for 3 minutes or until onion softens slightly.
Return the chicken to the pan with fajita seasoning. Cook, stirring, for 2 minutes or until aromatic. Remove from heat.
Heat tortillas following packet directions. Place tortillas on serving plates. Top with chicken mixture and dollop with avocado. Roll to enclose filling. Serve immediately with mixed salad leaves, if desired.
500g green prawns, peeled and deveined, can be used as a seafood alternative to the chicken.
Serving Size 4