Peel and clean the cucumber, cut it in half lengthwise, remove the seeds and cut into cubes. Clean and wash the leek and cut into fine strips. Wash fish, dab dry and cut into 12 pieces of as equal a size as possible.
Heat 1 tablespoon of oil. Add the cucumber and leek and fry until translucent, turning. Drain the peppers, cut them into strips and add them. Dust with flour, add stock and milk, bring to the boil while stirring and simmer for about 3 minutes.
Peel and grate the horseradish. Season sauce with horseradish, salt and pepper. Drain the silver onions, add them and warm them up in the sauce. Refine with cream. Meanwhile, prepare rice in boiling salted water according to package instructions.
Season fish with salt and pepper. Heat 2 tablespoons of oil in a frying pan and fry the fish for about 4 minutes while turning. Arrange rice, vegetables and fish on plates.