Prep the chicken. Use leftover roasted chicken or pull the meat from a rotisserie chicken.
Chop or shred into bite-size pieces. Aim for 2 1/2 to 3 cups.
Mash the avocado base. In a large bowl, scoop in two ripe avocados. Add 1/3 cup Greek yogurt, 1 to 2 teaspoons Dijon, the juice of half a lemon, and a pinch of salt and pepper.
Mash with a fork until mostly smooth with a little texture.
Add aromatics and crunch. Stir in the celery, red onion, and garlic. Fold in chopped herbs. Taste and adjust with more lemon, salt, or Dijon as needed.
Combine with chicken. Add the chicken to the bowl and gently fold until everything is coated.
If it looks dry, splash in a bit more lemon juice or an extra spoonful of yogurt.
Toast the bread. Lightly toast slices of your favorite bread. This keeps the sandwich sturdy and adds a nice crunch.
Assemble. Layer lettuce on the bottom slice to create a barrier. Pile on the avocado chicken salad.
Add extras like tomato slices or cucumber if you like. Top with the second slice of bread.
Serve. Slice in half and serve right away. A side of crisp apples, kettle chips, or a simple salad pairs well.