Peel, wash and chop the potatoes. Heat 2 tablespoons of oil. Fry the potatoes for about 15 minutes. Turn from time to time. Heat 1 tablespoon of oil in a coated pan. Fry the fish fingers in it at medium heat for about 10 minutes. In the meantime peel and chop the onion.
Cut cucumbers into slices. Add the onion and cucumber to the potatoes 5 minutes before the end of the frying time. Season with salt and pepper. Bring stock and milk to the boil, stir in mustard, thicken with sauce thickener. Season to taste with salt and pepper. Serve fish fingers, potatoes and sauce together. Serve with parsley and tomato slice