Step 1

Combine mascarpone and watercress in a food processor and whiz until a coarse paste. Season and set aside.

Step 2

Cut salmon into 2 equal-sized pieces. Roll out one of the pastry blocks on a lightly floured surface to a 20cm x 30cm rectangle. Lay one piece of salmon on top of the pastry, spread with half the mascarpone mixture and lay remaining salmon on top. Roll out remaining pastry block on a lightly floured surface to a rectangle slightly larger than the first, then place over the salmon, pressing the edges together to seal. Trim excess, fold in the ends and sides, and chill for 15 minutes.

Step 3

Preheat the oven to 180°C. For the lemon butter sauce, place all ingredients in a blender and blend until smooth. Loosen with 1 tablespoon hot water.

Step 4

Place the salmon en croute on a baking tray and brush the pastry with egg. Bake for 20-25 minutes until pastry is golden and cooked. Rest for 5 minutes, then slice and serve with lemon sauce.