Cook the eggs: If you don’t have hard-boiled eggs ready, place eggs in a pot, cover with an inch of water, bring to a gentle boil, then cover and simmer 10 minutes. Transfer to an ice bath for at least 5 minutes.
Peel when cool.
Prep the mix-ins: Finely chop celery, onion, and herbs. Keep pieces small so they blend well into the salad.
Make the dressing: In a large bowl, stir together lemon juice, Dijon, and mayo or yogurt (if using). Season with a pinch of salt and pepper.
Add the avocado: Dice the avocado and gently fold it into the dressing.
Coat well to prevent browning.
Add the eggs: Chop the eggs into medium pieces so they stay chunky. Fold them into the bowl with the celery, onion, and herbs.
Season and adjust: Taste and add more lemon, salt, or pepper as needed. If you like extra zip, add smoked paprika or hot sauce.
Serve: Spoon onto toasted sourdough, stuff into lettuce cups or pita, or serve over mixed greens.
Add capers on top for a briny finish if you like.