Firstly if you don't have leftover mashed potato start by making this (note this will extend the preparation time). Peel the potato and cut into large dice. Place in boiling salted water and cook until soft (15-20 minutes).
Drain the potato and leave to steam for a minute to remove some of the moisture. Mash or press through a potato ricer (such an amazing invention!) Add a knob of butter, a splash of milk and some salt and pepper and leave to one side to cool down.
Prepare 3 bowls; one with the beaten egg, one with a few tablespoons of plain flour and one with the breadcrumbs.
Mix the crab with the mashed potato and shape into little patties. You should have 6 depending how big you make them! Dip first into the flour, dust off the excess, dip into the egg then into the breadcrumbs, reshape and put on a plate/tray. I wear latex gloves for this as it can be messy!
If you have time or are preparing in advance these can go into the fridge to chill.
When you are ready to cook the crab cakes turn the oven to 200 degrees and put a splash of oil in a non stick frying pan. Cook the cakes for a minute or so on each side until starting to turn golden then place on a baking tray and cook in the oven for around 15 minutes or until golden and crispy.
If making the mayonnaise whisk the egg yolk in a bowl with a pinch of salt. Add the sunflower oil drop by drop, whisking continuously until thick (you may not need all the oil). Then whisk in the olive oil gradually until its all incorporated. Add lemon juice to taste and tarragon if using.
Cook the asparagus in boiling water for around 3 minutes until tender.
To serve put some mayonnaise on each plate, top with the asparagus, the crab cakes and add a lemon wedge if you like.
