Halve, pit, and peel the avocado and mash it in a medium bowl until mostly smooth.
Add the Greek yogurt, lemon juice, dill, garlic powder, diced green onion, salt, and a pinch of pepper to the mashed avocado and stir until combined.
Gently fold the cooked, coarsely chopped chicken, chopped celery, and chopped almonds into the avocado-yogurt mixture until evenly coated.
Prepare the ciabatta rolls; toast them if desired. Place lettuce and sliced tomato on each roll.
Divide the chicken salad evenly among the four rolls, assemble the sandwiches, and serve.
