Thinly slice the cucumber, halve the cherry tomatoes, finely slice the red onion, and chop the fresh parsley. Place everything in a large mixing bowl.
Shred or dice the cooked chicken breast and add it to the bowl with the vegetables.
In a small bowl, combine the olive oil, lemon juice, minced garlic, salt, and black pepper. Stir well until mixed.
Pour the dressing over the chicken and vegetables. Gently toss everything together until all the ingredients are evenly coated.
Taste the salad and add a little more salt, black pepper, or lemon juice if needed.
Transfer the salad to serving bowls or plates. Sprinkle the chopped parsley on top and serve immediately.