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).
