Using your fingertips, rub the pesto into the chicken on both sides. Cover and chill for at least 1 hour.
With a sharp knife make a cut into the chicken breast lengthways and cut to open like a book. Place a layer of cling film over top and bash the chicken with a rolling pin to flatten out into an escalope. Repeat.
Heat the chargriller or BBQ and cook the chicken until cooked, about 5 minutes each side depending on the size. Squeeze a little lemon juice over and seasoning.
Plate up with the chicken first. Top with rocket, onion and Parmesan shavings. Pour over a little olive oil and balsamic. Serve.