Slice the cucumber finely, put in a colander and sprinkle over 1 tsp salt. Leave to stand for 45 minutes to draw out any bitter juices, then rinse briefly.
Meanwhile steam the chopped fennel for 10 minutes until tender to the point of a knife. Heat the butter in a pan and add spices and toast for one minute. Add the cucumber and fennel. Sweat for around 10 minutes. Pour in the stock, bring up to the boil, then switch off the heat and leave to cool. Once cool, liquidise in a blender with the yogurt, mint and a little salt and pepper.
Pour into a bowl and chill for at least three hours. Adjust the seasoning before serving, then garnish with the fennel fronds.