Preheat the oven to 200°C.
Roll the pastry to 3mm thickness on a floured surface and cut two rectangles measuring 30cm x 10cm and 37.5cm x 15cm.
Trim the salmon into a 27.5cm x 7.5cm rectangle.
Purée the salmon trimmings and egg whites in a food processor then blend with 50ml cream and seasoning.
Spread the salmon purée onto the smaller pastry piece and top with langoustines and seasoning.
Layer spinach and the salmon piece on top, cover with the larger pastry, crimp edges, and brush with egg yolks.
Bake for 30-35 minutes until golden and rest for five minutes.
Sauté butter, onion, carrots, fennel, and thyme until softened.
Stir in langoustine shells, cayenne, and tomato purée then reduce the white wine by half.
Add fish stock, boil, and simmer for 15 minutes.
Blend the sauce until smooth, sieve into a pan, add cream, and simmer with lemon juice and seasoning.
Carve the tart and serve with the sauce.