1. Preheat the oven to 180°C. Halve the pastry sheets to create six rectangles of even size. For the tart base, stack four rectangles on top of each other and sandwich between two pieces of baking paper. Roll the sheets out as evenly as possible to a 15 centimetre x 30 centimetre rectangle around 4 millimetres thick, trimming the edges where necessary. Place onto a large lined baking tray.

2. Halve the remaining two pastry rectangles to create four long strips. Stack the strips on top of each other and roll out between two pieces of baking paper to four millimetres thick. Cut the pastry into three centimetre wide strips that you can use to create an edge on your tart base.

3. Brush the tart base with egg and lay the three centimetre strips on top to create an edge around the base. Brush the strips with egg. Prick the base with a fork and score the inner perimeter of the edge with a sharp knife, this will allow the edges to rise. Weigh the base of the pastry with baking beads or uncooked rice and bake in the oven for 20 minutes, then remove the beans and bake for a further 25-30 minutes or until base is golden and cooked through. Remove and set aside.

4. Bring the stock to the boil with the bay leaves, peppercorns, salt and two cups of water in a medium saucepan. Blanch the asparagus in the stock for 2-3 minutes until almost tender. Remove with a slotted spoon and refresh under cold running water. Set aside.

5. Return the stock to the boil, add the chicken breasts, turn off the heat and cover with a lid. Allow to stand for 12 minutes then remove and set aside for five minutes to rest, reserving the stock. Thickly shred the chicken.

6. Melt the butter in a separate saucepan over medium heat. Add the flour and cook, stirring, for one minute to cook the flour. Gradually add 1½ cups (375 millilitres) of the stock, whisking constantly, then the milk until smooth and thickened slightly. Add salt to taste and the shredded chicken and asparagus and stir until warmed through.

7. Spoon the chicken and asparagus mixture into the pastry case. Scatter with herbs and watercress. Serve immediately.