Halve the avocados and remove the seeds. Use a spoon and take 1/4 of the avocado flesh out of each avocado. Set the scooped avocado flesh aside for the chicken salad. This will also make the hole bigger so you can fill it with the chicken salad.
Cook chicken breast by boiling or baking it (can use chicken thighs) and cut it into small strips
Peel and finely slice the onions, tomato and celery.
To make the chicken salad: add the chicken, mayonnaise, tomato, onion, celery and mashed avocado to a mixing bowl. Mix well.
Scoop this salad mix into the avocado halves to fill the pit holes.
Squeeze in fresh lime juice if you want and season with salt and pepper. Drizzle olive oil on top if you want to. Serve on a bed of lettuce.