If starting with raw chicken, season it with salt and pepper, then grill or bake until cooked through (internal temperature of 165°F or 74°C). Let it cool, then dice into small cubes.
Cut the avocados in half, remove the pit, and scoop the flesh into a large bowl.
Mash the avocado with a fork until it reaches your desired consistency (smooth or slightly chunky).
Add the diced chicken, halved cherry or grape tomatoes, red onion, celery, and red bell pepper to the bowl with the mashed avocado.
Sprinkle in the chopped cilantro, lime juice, garlic powder, salt, and pepper.
If using, add the Greek yogurt for extra creaminess.
Gently fold all ingredients together until well combined.
Taste and adjust seasoning as needed, adding more lime juice, salt, or pepper to your preference.
Serve immediately or chill in the refrigerator for at least 30 minutes to allow flavors to meld.