Mix the gelatine powder with a little bit of water and let it stand until all the water is absorbed.
Liquidize 400 g cucumber and mix it together with the lime or lemon juice, chili flakes, sour cream, tarragon, salt and pepper.
Warm the gelatine in the microwave for 10 or 20 seconds until it ibecomes liquified.
Mix 3 tablespoons of the cucumber mixture into the gelatine and stir well.
Now you can add the gelatine into the rest of the cucumber mixture.
Put the cucumber mix into the fridge until it just starts to set. Then mix in the Greek joghurt or the beaten cream.
Fill the mousse into serving glasses and let it set in the fridge for at least 4 hours or overnight.
Decorate the mousse with the radish, cucumber cubes, dill or fennel fronds and smoked fish if you are using it.