Cut celery thin and boil in salty water with lemon juice until soft. Put in a colander and drain.

Cut onion and garlic into rings and fry in butter until transparent. Mix in cream and stew 5 minutes. Salt, pepper.

Grate horse-radish root finely and force together with butter through a sieve.

Clean and bone fish, discard skin, and cut into slices.

When serving, put warm vegetable mousse on a plate, spread out fish slices alongside.