Boil and mash the potato, adding in the butter and milk to create a creamy texture. Stir in the chives and season with salt and pepper.
Drain and empty the tuna into a bowl. Add in the mayonnaise and season with salt and pepper.
Place the rocket leaves in the centre of a plate. Put a metal ring (approx. 8cm diameter) on top and add two tablespoons of mashed potato (approx 2cm high) pushing it down firmly. Add on top, the tuna mix and then repeat the process with the mash potato.
Decorate the top, crossing two anchovies. Heat the olive oil and balsamic in a pan seasoning with salt and pepper. Once thickened drizzle over the rocket leaves and serve immediately.