Cut the chicken breasts widthways into very thin pieces.
Heat 5 tablespoons olive oil in a large pan over medium heat.
Add the finely chopped onion and cook for 5 minutes until soft but not brown.
Add the finely chopped garlic and cook for 30 seconds.
Add the chicken pieces to the pan and cook on medium heat for 5 to 10 minutes, stirring occasionally, until the chicken is cooked through (internal temperature 74°C / 165°F) and light golden brown.
Add the lemon juice and grated lemon zest to the pan.
Increase heat to medium-high and bring to a gentle simmer.
Stir everything with a wooden spoon, then remove the pan from heat.
Stir in the 4 tablespoons of freshly chopped parsley and season with salt and pepper to taste.
Transfer the chicken mixture to a serving bowl while still hot.