For the tuna mousse, drain the tuna from the can. Soak the gelatin sheets in water. Peel and chop the onion, mix with the tuna and the crème fraiche.
Heat the lemon juice, squeeze the gelatine leaves, dissolve in it and mix into the mixture. Fold the whipped cream into the tuna mixture and season with salt and pepper.
Pour the mixture into a mold lined with plastic wrap and smooth out. Close the cling film over the tuna mousse and refrigerate for some time.