Bring a pot of water to a boil and cook the eggs for nine minutes, so the yolks are completely set but still creamy. Transfer them to cold running water to cool rapidly, then peel them; shocking them in cold water not only makes peeling easier, but also prevents the grayish-green ring around the yolk.
Halve the avocado, remove the pit, and mash the flesh in a bowl with the yogurt, mustard, and lemon juice into a coarse cream. Leave some chunks in it, as it has a nicer texture than a smooth puree.
Roughly chop the peeled eggs and gently fold them into the avocado cream. Season generously with salt and black pepper, and stir in a large handful of finely chopped chives.
Slice the cucumber diagonally into thick slices of nearly one centimeter and arrange them on a plate. Spoon the egg salad on top, drizzle with the olive oil, and sprinkle with the black sesame seeds. Three eggs with a whole avocado provide over twenty grams of protein and plenty of monounsaturated fats, while the cucumber serves as a crunchy base without adding carbs.