Grind the fish twice through a meat grinder, adding boiled potato, salt and flour as you do so. This gives the mixture a somewhat sticky consistency. (You can also use a food processor, although this will make it more difficult to get an even consistency.)
Work the mixture together, gradually adding milk. Do not add too much milk at once, no more than half a cup at a time, carefully working the mixture together each time.
Continue until all the milk is used and the mixture has a smooth texture. Add eggs. Work rapidly, so that the mixture doesn’t warm up too much. If it does, leave to rest in refrigerator. Add nutmeg and a good pinch of salt and pepper and mix well.
Transfer the mixture to three separate bowls to make differently flavored fish cakes. To one bowl, add chopped herbs and half the garlic. To the other add spring onion, beetroot and the other half of the garlic. To the third, add cayenne and paprika. Mix each batch thoroughly.
Using a spoon and your hands drenched in cold water, mould the mixture into cake-like balls, each approximately the size of a plum. In a skillet, heat the butter and fry the cakes until golden brown on all sides, 4 to 5 minutes.