Wash the lemon and grate dry. Tear off half of the peel in zests and place in water. Halve the lemon. Squeeze juice from 1/2 lemon. Cut avocado in half, remove stone and remove flesh from skin with a spoon.
Crush the flesh finely with a fork and season to taste with salt, pepper and lemon juice. Peel the grapefruit so that the white skin is completely removed. Remove the fillets with a sharp knife between the parting skins.
Halve the salmon slices. Wrap 1 grapefruit fillet each in 1/2 salmon slice. Cut baguette diagonally into thin slices and spread with avocado cream. Place salmon-grapefruit fillets on top. Garnish with lemon zests and sprinkle with coarse pepper.