Prepare classic mashed potatoes. Spread in a thin layer on a plate. Place thinly sliced onion and brush with oil.
Remove all the bones from the fish and cut into slices. Put in vegetable oil for 30 minutes so that the fish gives up unnecessary salt). Pull out, blot with a napkin.
Cut the celery stalk into slices in length and put on mashed potatoes. Garnish with spinach and halibut caviar.
Peel the celery root and cut into cubes 1 by 1 cm and cook over low heat until soft. Punch in a blender, add salt, olive oil, strain through a sieve. Put on a plate around the edges of the fish.
Beat the protein until a white thick foam, add agar-agar (3 g), honey and juice. Mix slowly with a whisk. Transfer the protein mass to a pastry bag and squeeze out small dots of 20 g each. Leave to dry for 4 hours at 40 degrees. Put the finished meringues into the celery puree.