Boil the eggs, let cool, and peel. Cut in half, remove the yolks and mash with a fork. Cut the egg whites into small cubes. Peel the onion, cut into small pieces and add to the egg yolk. Add the spices, basil, mustard and creme fraiche. Stir until smooth. Add the egg whites, mix and allowed to stand in a tightly closed bowl for a couple of hours in the fridge to allow flavors to blend.
Also known as: Savory Egg Cream Sandwich With Egg, Cooked Salted Egg Cream