Peel the avocado and mango and cut into small cubes. Peel the shallot and cut it into small cubes.
Mix the mango, avocado, olive oil and shallot together. Season to taste with salt, pepper, sugar and possibly a few squirts of lemon juice.
Put a metal ring on a plate. Pour in the tartar, press it down and carefully peel off.
Heat the butter in a pan and fry the prawns over medium heat. Season to taste with salt and lemon juice. Place the prawns on the tartare and drizzle with the lemon butter.