Chicken pot pie - Good Food Middle East

Ingredients

• 2 tbsp vegetable oil
• 8-10 skinless, boneless chicken thighs (about 1kg)
• 2 onions, finely chopped
• 2 celery sticks, finely chopped
• 2 carrots, finely chopped
• 50g butter
• 50g plain flour
• 500ml milk
• 1 chicken stock cube
• 1 tbsp Dijon mustard
• small bunch of thyme, leaves picked, or use 1 tsp dried thyme
• 1 egg, beaten
• 375g sheet ready-rolled puff pastry