Mackerel on Toast with Salted Cucumber and Horseradish

• 1 medium mackerel, filleted
• large-flaked sea salt and black pepper
• butter

Skin the cucumber and slice on a grater on the single-blade side or on a mandoline. If you are chopping the slices, they must be cut very thin. Put the slices in a sieve or colander and throw in enough salt to make you think you have added a little too much. This is correct. Mix in the salt well and leave the contents to drip over the sink for half an hour. Then quickly run a little cold water