Melt margarine in a frying pan over moderate heat, add spinach and watercress and toss for 1 minute.
Transfer mixture to a bowl and chill.
Puree fish in a food processor or blender for 1 minute.
Add egg white and process for 30 seconds.
Add milk gradually while processor is running, then add spinach and watercress, process for 1 minute.
Place mousse mixture in a bowl, cover and chill.
Remove cover and mix in nutmeg, chives and lime juice.
Cut 8 foil rectangles, about 25x13cm and lightly grease.
Place teaspoons of mixture on each foil rectangle, roll up into a sausage shape.
In a large frying pan heat enough water to cover foil sausages.
When simmering, add sausages and cook for 6 minutes.
Remove with a slotted spoon, set aside.
Puree tomatoes in a food processor or blender.
Heat tomatoes, tomato puree and Tabasco in a saucepan over moderate heat, until warned through.
Pour sauce on each serving plate top with sausages.