Cook the egg for about 10 minutes. Wash and peel the asparagus and cut off the woody ends. Bring water to the boil in a wide shallow pot and season with salt, butter and a little sugar. Cook the asparagus for 13-15 minutes.
Quench the egg and let it cool down. Cut the gherkins into small cubes. Peel and finely dice onion. Wash parsley, dab dry and chop the leaves finely. Finely chop 4 capers. Halve the remaining capers.
Peel and chop the egg. Mix parsley, gherkins, onion, egg, mayonnaise and yoghurt and season to taste with salt, pepper and possibly some cucumber stock. Wash, clean and drain the rocket.
Cut the roll 3/4 lengthwise. First spread rocket, then asparagus on top. Put ham slices on top or between the asparagus spears. At the end spread some sauce tartare and half caper apples on the burgers.
Add the rest of the tartar sauce.