Make the hard-boiled eggs: Fill a saucepan with 1 inch of water and insert a steamer basket. (If you don't have a steamer basket, that's ok.) Bring the water to a boil, and gently place 6 eggs in the steamer basket or directly in the saucepan. Cover the pot and steam the eggs for 15 minutes. Remove the eggs and place them immediately in ice-cold water to cool.
Once they've cooled, carefully peel the hard-boiled eggs and cut them in half lengthwise. Place them on a serving platter. Scoop out the cooked yolks and set them aside in a mixing bowl.
Cut the avocados in half. Remove the pit. Scoop out the avocado flesh and place it in the bowl with the egg yolks.
Roughly mash with a fork. Mix in the Greek yogurt, feta, cilantro, chile pepper, and chives and mash until smooth.
Spoon a generous spoonful of the avocado mixture into each well of the hard-boiled egg whites. Garnish with chopped cilantro or parsley. Serve.