In a medium bowl, add the scooped avocado flesh. Mash with a fork until mostly smooth but with some small chunks remaining for texture.
Add the chopped hard-boiled eggs, Greek yogurt (or mayo), lemon juice, Dijon mustard (if using), diced red onion, chopped dill, chives, garlic powder, salt, and pepper to the bowl with the avocado.
Gently fold all ingredients together until well combined. Taste and adjust seasoning with more salt, pepper, or lemon juice as needed.
Spoon the avocado egg salad generously onto slices of bread. Garnish with extra fresh dill, chives, or other optional garnishes like microgreens. Serve immediately.