Cut avocado and nashi into ½ cm (¼ in) cubes and mix with parsley, oil and juice. Season with salt and pepper. Sit a biscuit cutter in the centre of a plate and gently press a quarter of the mixture inside it to make a flat disc. Run a knife around the inside of the cutter and lift it up. Do the same on three more plates. Divide the prawns into four and sit a pile on each disc. Dollop some mayonnaise on top.