Food processors have made it possible to produce lovely, light fish mousselines, as well as smooth terrines without long arduous pounding and beating by hand with a wooden spoon. But if you want your purée to be extra smooth, you can press it through a sieve or pass it through a food mill.
Most firm fish, such as sole, haddock and salmon, is suitable, although oily fish like herrings and mackerel are too rich and some shellfish, such as scallops, too watery. The other ingredients are simply double cream, egg white and lots of seasoning, ideally with sea salt and white pepper and maybe a good pinch of paprika. As it is difficult to season the mousseline after cooking, check by cooking a small spoonful of the mixture first, by poaching in alittlestock or water.
The secret of a light mousseline is to keep the mixture well chilled while you beat in the cream. This helps the mixture maintain a firm texture and yet cook to a delicate lightness. Be sure to have crushed ice ready to use in step 2. Mousselines can be cooked simply in buttered moulds or ramekins, with a delicious surprise base of chopped, lightly cooked asparagus, tomatoes, peppers or mushrooms, if you like.
Trim 500g (1 lb) skinless, fish fillet of membranes and stray bones. Cut into cubes, then whizz in a food processor until smooth. Add 2egg whites and season more generously than you would think. Process until well mixed and spoon into a bowl.
Place the bowl in a larger one containing crushed ice cubes, then gradually beat in 200–300ml (7–10fl oz) chilled double cream, depending on the fish; firmer fish need more cream. The mixture should be thick and smooth and hold its shape.
Check the seasoning by poaching a spoonful in simmering water for 3 minutes and tasting. Spoon the mixture into a 900ml (1½pint) ovenproof dish or into 6 individual 150ml (1¼pint) ramekins, on top of a base of lightly cooked vegetables if liked, and cover with greased foil.
Cook in a bain-marie at 180°C, 350°F, Gas Mark 4 for 25–30 minutes until just firm in the centre. Run a knife round the outside and unmould on to small serving plates. Serve warm with hollandaise or other hot buttery sauce.