Chicken Pie with Puff Pastry | Easy Chicken and Vegetable Pot Pie

Ingredients

• 1.5kg chicken (3lb 5oz)
• Salt flakes and freshly ground black pepper, to taste
• 75g butter (2 1/2oz)
• 1 brown onion, finely chopped
• 1 leek, white and pale green part, washed well and finely sliced
• 2 cobs corn, kernels removed
• 2 carrots (about 300g/10 1/2oz), peeled and finely chopped
• 180g shelled green peas (6 1/2oz) (about 500g/1lb 2oz peas in the pod)
• 2 tablespoons plain flour (25g/1oz)
• 1/2 teaspoon mustard powder
• 1/3 cup milk (80ml)
• 1/3 cup cream (80ml)
• 1 bunch chives, chopped
• 7 sprigs thyme, leaves only
• 2 sheets butter puff pastry
• 1 egg, lightly beaten
• Green salad, for serving