Put the cold mashed potatoes in a mixing bowl, add the mayonnaise, 2 tbsps of lemon juice, pinch of salt and mix well. Put the diced avocado in a small bowl and sprinkle with lemon juice. Add the onions, the shredded tuna and season with salt and cayenne pepper.
Line 4 small bowls with cling film. Divide half the mashed potatoes and place in the bottom, make a layer with the tuna mixture and top with the remaining potatoes. Refrigerate for at least 2 hours.
Unmould onto serving plates and decorate with the hard boiled eggs, avocado slices, black olives and a sprinkling of cayenne. Sprinkle the top with some oil.