Slice one chicken breast horizontally, but not all the way through. Open chicken out and cover with a piece of cling film. Pound with a meat mallet or rolling pin until evenly thin. Repeat with the other three chicken breasts.
Coat the chicken with pesto. Cover and chill for 30 minutes to 1 hour.
Meanwhile, preheat the barbecue.
Drizzle chicken with olive oil. Place chicken breasts on the hottest part of the barbecue and sear for 2-3 minutes. Turn over.
Spoon some more pesto on the chicken. Top each piece with a slice of cheese, tomato and another slice of cheese. Cook for a further 2-3 minutes or until chicken is cooked through.