For the relish, peel onion and garlic. Wash and clean the cucumber. Cut onion, garlic and cucumber into fine cubes. Mix prepared ingredients, vinegar, sugar and salt in a pot and cook over high heat for about 5 minutes.
Allow to cool briefly and chill.
For the polenta, boil milk and 750 ml water. Add butter and about 1 teaspoon of salt. Stir in semolina. Continue stirring over low heat for 5 minutes. Cook for another 15 minutes on a switched off hotplate.
Wash the fish, dab dry and season with salt and pepper. Heat oil in a pan, fry fish on each side for about 2 minutes. Arrange polenta, fish and relish on plates. Garnish with dill.