Preheat oven to 180°C.
Heat 1 tbs of olive oil in a frypan. Brown the chicken in batches.
Remove chicken from pan and set aside.
Heat remaining oil, add the leeks, and sauté for 5 minutes. Stir the flour through the leeks and cook for 1 minute. Whisk the stock into the leek mixture to make a smooth sauce; continue to stir as you add the mustard and verjuice. Stir in peas. Season with salt, pepper, and thyme leaves; remove from heat.
Divide the chicken evenly into 4 small pie dishes. Spoon sauce over the chicken. Cut the pastry into quarters and cover the top of each pie dish with pastry, pressing down around the edges. Using a sharp knife, make a cross incision in the centre of each pastry lid. Brush the pastry with egg.
Transfer pie dishes to a baking tray and bake in oven for 25 minutes.