Chicken Pot Pie

• 1 tbsp extra virgin olive oil
• 1 brown onion, diced
• 2 celery sticks, diced
• 2 carrots, diced
• 1 tbsp fresh thyme, finely chopped
• 500 g chicken thigh fillets, diced
• 2 tbsp plain flour
• 1 cup chicken broth
• 1 cup milk
• 1.5 cup peas and corn, frozen
• 1 sheet puff pastry