In the bowl of a blender, add the fish fillet, the egg, the onion, the breadcrumbs, seasoning with salt and black pepper to taste.
Form the cutlets, place them on a greased baking sheet and bake in a preheated oven at 200°C for 8–10 minutes.
In the meantime we cook the sauce.
In a well-heated saucepan, melt the butter, add flour, stir for 30-40 seconds, pour in water, stir until lumps form, add sour cream, season with salt and black pepper to taste.
Mix and boil for 2-3 minutes.
Pour the sauce over the cutlets and reduce the temperature to 180°C.
