Halve avocado and remove the seed. Use a spoon and take 1/4 of the avocado flesh out of each. Set the scooped avocado flesh aside for the chicken salad. This will also make the hole bigger so you can fill the chicken salad into the pit hole.
Cook chicken breast by boiling or baking it (you can use chicken thigh) 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 the mixing bowl. Mix well.
Scoop this salad mix into the avocado halves to fill the pit holes.
Squeeze in fresh lemon juice and season with salt and pepper. Drizzle olive oil on top if you want to. Serve on a bed of lettuce.