Cook the octopus. Slice it and keep it in the fridge.
Finely chop the shallot, the coriander, the ginger and the lemongrass and reserve.
Prepare the roquette juice by mixing it with the sparkling water and salt. Work in the olive oil and reserve.
Season the octopus with the shallot, ginger, lemongrass, wild fennel and capers.