Put the salmon fillets in a lightly oiled shallow ovenproof dish and drizzle with the olive oil. Season lightly with salt and pepper, and place the dish on the upper rack of the halogen oven.
Set the temperature to 225°C (437°F) and bake for 10 minutes or until just cooked through. (Test by piercing the thickest area with the tip of a knife – the flesh should flake easily.)
Meanwhile, melt the butter in a saucepan until bubbling. Add the shallots and fry gently for 3 minutes. Sprinkle in the flour and cook, stirring, for 1 minute. Remove from the heat and gradually blend in the stock, then the watercress. Return to the heat and cook gently for 2 minutes, stirring, until thickened.
Pour the sauce into a food processor and process until smooth, or push through a sieve. Return to the rinsed-out pan and stir in the cream, nutmeg and salt and pepper to taste. Serve with the salmon, garnished with watercress sprigs, and accompany with buttered new potatoes.