Preheat the oven to 160°C.
Cook chicken legs, celery, and bay leaves in wine and stock for 30–40 minutes.
Separate meat from bones and strain the cooking liquid to reserve 750ml.
Fry bacon for 3 minutes, add onion and cook for 10 minutes until soft.
Wisk butter and flour into a roux and slowly add liquid to create a sauce.
Mix in crème fraîche, mustard, chicken, and celery, then transfer to a dish and chill for 30 minutes.
Roll pastry to 3mm thick, cut into 3cm ribbons, weave into a lattice, and chill.
Place the lattice over the filling, seal the edges, and brush with beaten egg.
Increase oven to 200°C and bake for 30 minutes until golden.