For the tart, preheat the oven to 200C/400F/Gas 6.
Roll the pastry out on a floured work surface to a 3mm/⅛in thickness. Cut out a 30cm x 10cm/12in x 4in rectangle and a 37.5cm x 15cm/15in x 6in rectangle.
Trim the salmon into a 27.5cm x 7.5cm/11in x 3in rectangle
Roughly chop the salmon trimmings (about 175g/6oz). Blend the salmon trimmings and egg whites to a purée in a food processor. Add the cream, salt and freshly ground black pepper and blend together until smooth.
Place the smaller piece of pastry onto a baking tray and spread over the salmon purée. Cover with the langoustines and season with salt and freshly ground black pepper.
Arrange the spinach leaves on top, then the piece of salmon. Cover with the larger piece of the pastry and crimp the edges. Trim the edges and brush with the beaten egg yolks. Score the pastry with a table knife.
Bake in the oven for 30-35 minutes, or until the pastry is cooked through and golden-brown. Set aside to rest for five minutes.
Meanwhile, for the sauce, heat a frying pan until hot and add the butter, onion, carrots, fennel and thyme and cook for 2-3 minutes, or until just softened but not coloured.
Add the langoustine shells, cayenne and tomato purée and cook for a further minute. Add the white wine and continue to cook until the volume of the liquid is reduced by half.
Add the fish stock, bring to the boil, then reduce the heat and simmer for 15 minutes.
Blend the sauce in a blender until smooth, then pass through a sieve into a saucepan. Return the sauce to a simmer, add the cream and cook for a further minute. Season, to taste, with salt, lemon juice and freshly ground black pepper.
To serve, carve the tart into slices and spoon over the sauce.