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Wrap the base of a loose-bottomed, 20cm non-stick cake tin with cling film, twizzling it into a knot on the underside, then slot base back in the tin. Cut the salmon into pieces about 15cm long and make a neat layer of slices, presentation side down, on the base of the tin. Start at the outside and push the slices right to the edge; after going all round the edge, fill in the centre with straight pieces of salmon.
Mash the cream cheese until smooth (a rubber spatula is good for this), working in the mignonette pepper and a pinch of salt at the same time. Pour in two-thirds of the cream, a little at a time, and keep stirring and beating until a smooth, lightly whipped cream consistency is achieved. Now work in the lemon juice. If slightly too firm (it should be spreadable), work in the remaining cream and then recheck for seasoning.
Dollop about a quarter of the cream cheese in the centre of the salmon and spread to the edge with the spatula or a palette knife; slowly turn the tin to get an even thickness of about 3mm. Top this with a salmon layer, continuing until both salmon and cheese have been used, finishing with a salmon layer. Press the top down with your hands. Stretch cling film over and refrigerate for at least four hours – or preferably overnight.
Make the dressing. Simmer the shallots, sugar and brandy in a small pan until almost dry and a deep brown colour – about 5 minutes. Tip into a bowl and leave to cool. Whisk the crème fraîche in a bowl with the mustard and lime juice, and whisk in the cold shallots.Whisk in the olive oil, trickling it slowly as if making mayonnaise. Season, cover the bowl with cling film and chill in the fridge for 2 hours or overnight.
To turn the salmon gateau out, dip a knife into a jug of warm water and run it carefully around the inside of the tin, between the gateau and the sides, to release the salmon from the tin. Invert a flat serving plate or board on top, then turn the gateau out upside down and lift off the tin and the cling-filmed base. You will now be left with a perfectly flat top to the salmon-layered gateau.
The gateau is best cut into wedges while set firm, then left to reach room temperature for 20-30 minutes before serving – it will soften slightly and have more flavour being less cold. Just before serving, chop the dill and whisk it into the dressing, which is best served straight from the fridge. Serve the gateau with watercress sprigs drizzled with olive oil and lime wedges for squeezing. Serve the dressing in a jug.