Grate the potatoes and squeeze as much water as you can out using kitchen roll or a tea towel until the potato is quite dry. Place in a bowl.
Chop the prawns into small pieces and combine with potato. Season well with sea salt and black pepper - set the rosti mix aside
Place the burgers onto the grill and cook according to packet instructions (18-20 minutes flipping regularly)
Heat a frying pan with the butter and oil, until the butter is frothy but not spitting. Split the rosti mixture into 4 equal portions and drop them into the frying pan (I used a circular metal cutter as a template). Using your spatula, shape until they are roughly the size of the burger buns and similar thickness.
Gently fry the rosti until a deep golden brown then using a spatula, carefully turn over and repeat the process until you have 4 crispy potato cakes.
Make the watercress mayonnaise by blitzing in a blender 40g of the watercress with 125g of the mayonnaise and a splash of lemon juice. (Finely chop and mix if no blender).
Fry the eggs to your preference (again I used the metal cutter for burger-sized egg), lightly toast the burger buns, spread watercress mayo over the bottom half of the burger bun and add a sprig of fresh watercress.
Place the burger on top, followed by the rosti, the fried egg and the bun top. Enjoy!