For the pommes purées, bring the potatoes to the boil in a large pan of cold, salted water. Reduce the heat until the water is just simmering and cook for 25-30 minutes, or until the potatoes are soft.
Drain well in a colander, setting the drained potatoes aside for 2-3 minutes to drive off the excess steam.
Mash the potatoes until smooth using a potato ricer or masher. Return the potato purée to a saucepan, add the milk and beat well until combined.
Beat in the butter until the mixture is fluffy and forms firm peaks, then season, to taste, with salt and freshly ground white pepper. Keep warm.
Meanwhile, for the pollock, preheat the oven to its highest setting.
Scatter the bread cubes onto a large baking tray and dry roast in the oven for 3-4 minutes, or until crisp and golden-brown. Remove the croûtons from the oven and set aside.
Reduce the oven temperature to 200C/400F/Gas 6.
Pat the pollock fillets dry using kitchen paper, then season all over, to taste, with the salt and freshly ground white pepper.
Heat the butter in a frying pan over a medium heat. When the butter is foaming and starting to brown, add the pollock fillets, skin-sides facing upwards, and fry for 4-5 minutes, or until two thirds of each fish fillet has turned opaque.
Using a fish slice, carefully turn each pollock fillet over and continue to cook for a further 2-3 minutes.
Transfer the fish fillets to a cold ovenproof frying pan and roast in the oven for 3-4 minutes, or until just cooked through, then set the fish fillets aside on a warm plate to rest.
Return the frying pan used to cook the pollock to a high heat. Add the brown chicken stock and water and bring to the boil.
Add the capers, chopped lemon segments, shallots, caper berries and two-thirds of each of the herbs. Stir well and continue to cook for 1-2 minutes, or until warmed through. Season, to taste, with salt and freshly ground white pepper.
To serve, spoon the pommes purées into the centre of each of 4 serving plates. Place one pollock fillet on top of each portion. Spoon over the caper and lemon sauce, then sprinkle over the croûtons and the remaining herbs.