Cheesy chicken pot pies

Ingredients

• 1 tbsp olive oil
• 1 leek, finely sliced
• 1 carrot, finely diced
• 150g button mushrooms, diced
• 2 celery sticks, finely diced
• 1 1/2 cups chicken stock
• 1 tsp fresh thyme leaves
• 1 tsp mustard powder
• 1 tbsp cornflour, mixed with a little water to form a paste
• 1/2 cup cream
• 1/2 cup tasty cheese, grated
• meat from breast and wings of a cooked chicken, diced
• 1 sheet frozen puff pastry, defrosted
• 1 egg, lightly beaten