8 flounder fillets
fat for greasing
350 ml (12 fl oz) milk
1 onion, finely chopped
1 carrot, finely chopped
1 celery stick, finely chopped
1 bouquet garni
salt and pepper
2 tbsp (1 oz) 25 g butter
3 tbsp (25 g) plain flour
4 oz (100 g) Gruyere cheese, grated
2 oz (50 g) Parmesan cheese, grated
1.25 ml (1/4 tsp) mustard powder
chervil sprigs to garnish
1. Grease a shallow flameproof dish. Combine the milk, vegetables and bouquet garni in a saucepan. Add salt and pepper to taste. Bring to the boil, lower the heat and simmer for 10 minutes. Set aside to cool.
2. Fold the flounder fillets in three, skin side inwards. Strain the flavoured milk into a deep frying pan and heat to simmering point. Add the fish and poach for 6-8 minutes or until the fish is cooked. Using a slotted spoon, transfer the fish to the prepared dish. Cover with buttered greaseproof paper and keep warm. Reserve the cooking liquid in a jug.
3. Melt the butter in a saucepan, add the flour and cook for 1 minute, stirring. Gradually add the reserved cooking liquid, whisking constantly until the sauce thickens.
4. Mix the cheeses and stir half the mixture into the sauce, with the mustard. Remove the buttered paper from the fish, pour the sauce over the top and sprinkle with the remaining cheese mixture. Brown briefly under a hot grill. Garnish and serve.
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