Sear surface of chicken in half butter, remove. Cook bacon 1 minute, then leek, celery and garlic 5 minutes. Deglaze with wine, melt remaining butter, then cook flour 1 minute. Add stock, then remaining Filling ingredients. Add chicken, thicken sauce (~5 minutes).

Cool filling 1 hour, assemble pie with double layer puff on rim, brush lid with yolk, bake 45 minutes 200°C/375°F (180°C fan).

Trim the reedy dark green ends and root off, then cut in half. Cut each piece in half lengthways. Remove the outer 5 to 6 layers (keep them together), lay them flat, cut in half lengthways then into 1.8cm / 3/4" squares. Cut the inner core into 2cm / 0.8" pieces.

Chicken - Melt half the butter in a large pan over high heat. Add the chicken, salt and pepper. Cook until the surface changes from pink to white (inside still raw), then remove into a bowl using a slotted spoon. Leave residual butter and chicken juices in the pan.

Sauté - In the same pan, cook the bacon for 1 minute. Add the leek, celery and garlic. Cook for 5 minutes until the celery is softened. Don't let the leek brown - lower the heat if needed.

Deglaze - Add the wine and simmer rapidly on high heat, stirring, until it is mostly evaporated.

Roux - Lower the heat to medium. Add the remaining butter. Once it melts, sprinkle the flour across the surface (don't dump in one place) and stir for 1 minute.

Creamy sauce - While stirring, slowly pour the stock in. The mixture will thicken into a thick sauce once combined. Then add the mustard, cream, thyme, bay leaves, salt and pepper. Stir to combine, then add the chicken including all the juices accumulated in the bowl.

Simmer - Increase the heat, stir to combine. Once it comes to a simmer, reduce the heat to medium high. Simmer for 5 minutes, stirring regularly, or until the sauce is a thick creamy consistency you want in the finished pie (ie sauce thickness now = thickness in the baked pie, see Note 7).

Cool - Remove the pan from the stove and cool for 1 hour. (You can also store in fridge for 3 - 4 days).

Preheat the oven to 200°C/375°F (180°C fan-forced). Use a 1.5 litre / 1.5 quart baking pan (Note 9) or 4 to 5 x ~350ml / 12 oz ramekins.

Connect puff pastry sheets (if needed) - Semi-thaw the puff pastry (Note 6). Brush the edge of one sheet with egg white, then overlap 0.5cm / 0.2" with the other sheet to connect them. Use the back of a spoon to "smear" the pastry to hide the seam and firmly connect.

Cut out the pie lid (I use the dish as a cutting guide).

Optional strips for extra puffy edges (Note 8): Cut strips using the residual pastry to line the rim of the dish (if your dish has a thick rim, like mine) and place on the rim of the dish.

Fill the pie dish with the cooled filling and smooth the surface. Brush the pastry strips on the rim with egg white (if not using strips, don't do this step) then cover with the puff pastry lid.

Brush the lid with egg yolk. Cut a small X in the middle.

Bake 45 minutes or until the surface is a deep golden brown. Rest 5 minutes then serve!