Personal pies are always a delight.
Preheat oven to 425 F (220 C).
Melt the butter in a large saucepan. Add onions and cook until soft. Add the flour and whisk in. Let bubble gently. Slowly add milk, whisking. Bring to a gentle boil and stir until thickened.
Gradually incorporate chicken stock, then add sage, thyme, garlic powder, dry mustard, salt, and pepper. Add chicken, celery, carrots, and peas. Let cook five minutes, stirring.
Remove from stove. Divide mixture between ramekins. Brush the rims of the ramekins with egg wash. Carefully unroll puff pastry. Cut out rounds from pastry that are about five centimeters larger than your ramekins. Place a round of dough over top of each ramekin. Slice an X in the middle of each pastry round so steam can escape. Brush with egg wash. Bake in the oven for 15-20 minutes, or until crust is golden.