Mix all ingredients, except the cucumber, in a food processor or mash them in a bowl using a fork. Season with salt, pepper and lemon juice. Cut the cucumber in sections the length of the apple corer. Trim away the ends.
Using the apple corer, scoop out the seeds. You can also use a thin sharp knife, for that matter. Fill the created cavity with the fish mousse and wrap the rounds in plastic wrap. Leave to cool and set for about an hour in the refrigerator.
Cut the cucumber into thick slices with a sharp knife and enjoy!
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