Chicken, mushroom and sweetcorn pie - Green Giant

Ingredients

• 2 tbsp olive oil
• 3 x 150g chestnut button mushrooms
• 2 garlic cloves, finely chopped
• 1 onion, chopped
• 600g chicken breast, chopped
• 340g tin Green giant sweetcorn, drained
• For the white sauce:
• 40g unsalted butter
• 40g plain flour
• 300ml semi skimmed milk
• 250ml chicken stock, made with ½ stock cube
• 2 tsp Dijon mustard
• A handful flat leaf parsley, chopped
• A little plain flour, for dusting
• 500g puff pastry
• 1 medium egg, beaten
• Deep frying pan or saute pan with lid
• Pie dish
• Pastry brush