Remove all the bones from the fillet and remove the skin. Cut the pulp into strips. Switch on the oven with the top grill function to preheat to a high temperature.
Pour flour, a little salt, and pepper into a thick-walled saucepan, add butter, and pour milk. Put the saucepan over medium heat, stirring occasionally, bring the mixture to a boil. Prepare the sauce, stirring constantly with a whisk, for about 2-3 minutes.
Put the fish in the hot sauce and add half of the cheese, stir the sauce until the cheese melts.
Put the fish and sauce in heat-resistant portion molds. Mix the remaining cheese with bread crumbs, sprinkle the fish in tins with this mixture.
Place the tins with fish under the preheated grill, cook until golden brown on top. Serve fish immediately.