Cook the potatoes in boiling salted water until tender, about 15 minutes.
In a small saucepan, heat 1/2 cup of the milk. Drain and mash the potatoes, beating in the warm milk and 1/2 of the butter. Stir in the parsley and season to taste. Keep warm.
While the potatoes are cooking, melt remaining butter and cook the onion 5 minutes until just softened. Stir in the flour and cook 1 minute. Gradually add remaining milk, stirring continually over a medium heat for 5 minutes until the sauce thickens.
Stir in the smoked fish and capers and season with salt and pepper. Pour the mixture into a 1 1/2 litre ovenproof dish. Spoon the mashed potatoes over the top and sprinkle with parmesan. Preheat the grill.
Place the pie under the grill and cook five minutes until the potatoes are golden brown.