Start with the sauce: whisk the egg yolks until pale. Whisk the cornflour into the lemon juice and slowly whisk the mixture into the egg yolks. Add, in a thin stream, a third of the stock to the egg mixture while constantly whisking. Add remaining stock. Transfer mixture to a small saucepan and heat gently. Whisk until mixture thickens enough to coat the back of a spoon. Remove from heat, season to taste and keep covered.
Season the fish and sprinkle with chopped parsley. Heat a splash of olive oil and add a nugget of butter in a pan until hot. Place fish fillets in the pan and reduce heat to medium. Cook the fillets on both sides for 2 – 3 minutes until the fish is just cooked right through. Cover and keep warm.
Beat the egg whites until stiff and fold into the egg mixture. Spoon over fish and serve.