Heat a griddle pan to a medium heat.
Drizzle the chicken fillet pieces with oil and place the chicken fillet on the grill.
Cook for 4-6 minutes on each side or until cooked through. Season well with salt, pepper. Remove from the griddle pan and rest for a 1 -2 minutes.
In the meantime, in a small bowl, combine the mayonnaise, harissa and olives.
Drizzle some oil on the burger buns and place on the griddle pan to toast.
When ready, spread some mayonnaise mixture on the buns and add some tomatoes, salad leaves and add the grilled chicken and burger bun.