To make the mousse, place the sea bass in a food processor and blend at high speed to a purée. Add the egg white and blend again. Scrape the purée out of the food processor and, using the back of a ladle, push the mixture through a coarse sieve into a bowl.
Season the purée with salt and freshly ground black pepper. Add the cream little by little, stirring well between each addition. The finished mixture should be smooth and of a dropping consistency.
Next make up eight little parcels of the mousse. For each parcel, place a square of cling film on your work surface and put a spoonful of mousse into the centre. Bring the four corners up and tie together firmly as if you were tying up a balloon. Set the parcels aside in the fridge. (They can be prepared up to a day ahead.)
To make the brown shrimp and tarragon sauce, combine the lemon juice, wine, vinegar and shallot in a small non-reactive saucepan and reduce by half. In another pan, melt the butter and allow it to bubble.
Put the egg yolks, sugar and a pinch of salt in a food processor and blend. With the processor running, strain in the hot reduced shallot mixture, followed slowly by the hot butter. When all the butter has been added, pour the sauce into a bowl. Stir in the brown shrimps. (The sauce can be made up to one hour ahead and left in a bowl set over warm, but not too hot, water.) Add the tarragon at the last minute.
When ready to serve, place the cling film parcels in the top of a double boiler and steam over a gentle heat for about ten minutes. The mousses are ready when they feel firm to the touch.
Meanwhile, preheat the grill to high and bring a large saucepan of water to the boil, ready for cooking the sea bass and samphire. Score the skin of the sea bass fillets, being careful not to cut too deeply into the flesh. Season the flesh side, then arrange, skin-side up, on an oiled baking tray. Brush the scored skin with olive oil. Grill for about four minutes or until the skin has blackened and the flesh is just cooked. There is no need to turn the fillets.
While the fish cooks, drop the samphire into the pan of boiling water and add the sugar. Boil for two minutes, then test a sprig to see if it is cooked (the fleshy end should slip off the stem easily). Drain the samphire thoroughly and add the butter.
When the mousses are cooked, remove from the steamer. Place some samphire in the centre of each ovenproof plate. Set the grilled fillets of sea bass, skin-side up, on the samphire, then carefully peel off the blackened skin. Snip open the little mousse parcels with scissors, slip off the cling film and place a fish mousse on top of each fillet. Stir the tarragon into the brown shrimp sauce and spoon a little over each mousse. Place the plates under the still hot grill briefly to glaze the sauce (watching carefully so it doesn't burn), then serve at once.