Soak the gelatine in ice-cold water.
For the eel stock, sweat the shallots, the chopped fennel and the white leeks in olive oil.
Add the skin and bones of the smoked eel and then add white wine and reduce to half.
Add the fish stock and leave to simmer for 30 minutes.
Strain through a cheesecloth and reduce to 700 ml.
Add the soaked gelatine and place on ice-cold water.
Beat the whipping cream to yoghurt thickness, add a small portion to the set mixture and mix until smooth.
Fold in the remaining cream.
Season to taste with salt and lemon zest.
Fill silicone moulds.
Leave to set in the refrigerator and freeze in the blast chiller to at least -20°C to be able to turn them out easily.