Preheat the oven to 200°C/gas 6.
Place the salmon in a shallow, ovenproof dish. Top each salmon steak with the sliced onion and carrots then season with salt and freshly ground pepper.
Pour over the fish stock and add the bay leaf, chopped parsley and bring to the boil.
Remove from the heat, cover with greaseproof paper then place in the oven for eight to ten minutes.
While the salmon is poaching make the mousseline sauce. Place the egg yolks, mustard and white wine vinegar in a blender and process until well-mixed.
With the blender on constantly, gradually pour in the melted butter.
Mix in the tarragon, fold in the whipped cream and season with salt and freshly ground pepper.
Once the salmon has cooked remove the steaks from the stock. Remove the centre bone from each salmon steak and peel off the skin.
Garnish each steak with the cooked onion, carrot, sliced cucmber, and a little of the fish stock. Serve at once with the mousseline sauce on the side.