Cook the potatoes, uncovered, in salted water for approx. 20 mins. until soft, drain very well, push through a sieve, add the truffle butter and beat until fluffy with a ladle. Add a little milk if necessary, remove the pan from the heat.
Pre-heat oven to 200°C. Heat the truffle butter in a pan. Sauté the shallot for approx. 1 min. Pour in the vegetable stock, bring the boil, reduce the heat and reduce to approx. 3 tbsp. Pour in the full cream, season with salt and pepper. Add the cod and salmon fillet and divide into the oven-proof ramekins. Top with the mashed potato.
for approx. 15 mins. in the centre of an oven preheated to 200°C. Remove from the oven and allow to cool a little.