Cut the eggs lengthwise and remove the yolks with a spoon. Use a fork to mash them and add Greek yogurt and mustard. Fill the egg halves with the yolk mixture.
Pulse sprats using a hand mixer or an immersion blender, adding about 2 teaspoons of oil from the can. Top the eggs with pureed sprats using a teaspoon or a pastry bag. Sprinkle with garden cress to garnish (optional). Enjoy!