Add chicken into the pan, season with salt and pepper. Seal and brown the chicken for about 2 minutes, turning once.
Add dijon mustard and flour. Then chicken stock and stir until sauce thickens. Lastly, fresh thyme leaves and heavy cream.
Season, mix in frozen peas. Transfer pie filling into a pie dish. Cover the pie dish with just thawed puff pastry and loosely tuck in the sides.