First make the Mornay Sauce. Put the cold milk into a saucepan with a few slices of carrot and onion, 3 or 4 peppercorns and a sprig of thyme and parsley. Bring to the boil, simmer for 4–5 minutes, remove from the heat and leave to infuse for 10 minutes if you have enough time. Strain out the vegetables, bring the milk back to the boil and thicken with roux to a light coating consistency. Add the mustard and two-thirds of the grated cheese; keep the remainder of the cheese for sprinkling over the top. Season with salt and freshly ground pepper, taste and correct the seasoning if necessary. Add optional parsley.
Next make the Buttered Crumbs. Melt the butter in a pan and stir in 4 ozs (110 g/2 cups) white breadcrumbs. Remove from the heat immediately and allow to cool.
Skin the fish and cut into portions: 6 ozs (170 g) for a main course, 3 ozs (85 g) for a starter. Season with salt and freshly ground pepper. Lay the pieces of fish in a lightly buttered ovenproof dish, coat with the Mornay Sauce, mix the remaining grated cheese with the Buttered Crumbs and sprinkle over the top. Pipe a ruff of fluffy Duchesse Potato around the edge if you want to have a whole meal in one dish.
Cook in a moderate oven, 180°C/350°F/regulo 4, for 25–30 minutes or until the fish is cooked through and the top is golden brown and crispy. If necessary, place under the grill for a minute or two before you serve, to brown the edges of the potato.