Mince the fish. Place it with some salt in a pan over ice and beat. Add the egg white and continue to beat for a few minutes more. Slowly add 1 pint (600 ml) of the double cream, beating all the time. The mousse resulting should be light and should just hold its own weight when dropped from a spoon. Season with salt and pepper. Butter 4 ramekin moulds and fill with the mousse. Cover each with a piece of buttered aluminium foil.
Place the ramekins side by side in a roasting tin and add boiling water to three-quarters of their height. Place in a slow oven, gas mark 3, 325°F (160°C), and allow to cook for 10 to 12 minutes, until slightly firm to the touch.
To prepare the garnish, trim the asparagus spears so that they are all 4 in. (10 cm) long. Quickly blanch them in boiling water and refresh in ice. Roll the salmon slices around the asparagus spears, leaving the top and bottom showing. Place the spears in a buttered dish and cover with fish stock. Poach for about 2 minutes and strain off the stock and reserve.
To make the sauce, reduce the white wine, fish stock and chervil stalks until almost evaporated. Add the remaining 5 fl oz (150 ml) of double cream and cook to a sauce consistency. Drop in a few knobs of butter and flavour with lemon juice and salt and pepper to taste.
Turn out the ramekins on to 4 plates with the asparagus spears. Cover the mousse with the sauce and place a spring of chervil on top of each one. Serve immediately.