How to Make It
Fill a bowl with ice water. Put the eggs in a medium saucepan and enough water to cover by 2 inches. Bring to a boil. Cover the pan, remove it from the heat and let stand for 10 minutes. Drain the eggs and immediately add them to the ice water bath until thoroughly chilled. Drain the eggs and peel them, then coarsely chop.
In a large bowl, whisk the yogurt with the olive oil, mayonnaise and mustard. Stir in the celery, onion and chopped tarragon. Fold in the chopped eggs and the avocados and season the egg salad with salt and pepper.