Place the curry paste, egg and pollock fillet into a food processor and pulse until smooth.
Place the pollock mixture into a bowl and add the fish sauce, sugar, cornflour, lime leaves, coriander and green beans. Mix well.
With floured hands, mould equal quantities of the fishcake mixture into fishcakes approximately 7.5cm/3in in diameter (you should be able to make 6-8 fishcakes.
Half-fill a deep, heavy-based pan with vegetable oil and heat until a breadcrumb sizzles and turns brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Add the fishcakes in batches and deep-fry for 5-6 minutes, until crisp and golden-brown. Remove with a slotted spoon and drain onto kitchen paper.
For the dipping sauce, place the water, vinegar and sugar into a non-reactive pan over a medium heat and bring to the boil. Stir until the sugar has dissolved, then remove from the heat and allow to cool.
Once cooled, add the garlic, cucumber, shallots and ginger. Mix well, then pour into a serving bowl.
To serve, place some watercress onto a large serving plate and top with the fishcakes. Serve the dipping sauce alongside.