Rinse and cut the rhubarb into pieces. Place them in a jar. Boil the cider vinegar with 30 cl of water, the sugar, salt, and fennel seeds. Simmer for 5 minutes, then remove from the heat. Pour into the jar, close, and let cool.
Cut the fish into small pieces and divide among the plates. Slice the fennel and rhubarb and add to the fish.
Add the pickles and a little juice. Drizzle with lime juice and olive oil. Garnish with fennel flowers.