Halve the avocado, remove the stone and peel the skin. Finely dice 1/2 avocado. Mix avocado and lime juice. Finely dice crab meat and salmon and mix into the avocado. Season with pepper.
Carefully mix in the oil. Place 1 ring (9 cm Ø) on each plate. Spread the tartar evenly in the ring and press down gently. Loosen the ring carefully with a knife. Cut 1/2 avocado into slices.
Add a dash of sour cream and caviar to the hors d'oeuvres. Sprinkle with pepper. Garnish with avocado slices, lime slices and dill flags.