Chicken spring onion mustard pie

Name: Chicken, spring onion & mustard pie
Ingredients: 1 tbsp olive oil; 75g smoked pancetta, finely chopped; 6-8 boneless, skinless chicken thighs, cut into bite-sized pieces; 300g mushrooms, quartered; 25g unsalted butter; 200g spring onions, sliced, white and green parts separated; 3 tbsp plain flour, plus extra for dusting; 800ml chicken stock; 1 tbsp Dijon mustard; 2 bay leaves; ¼ tsp ground nutmeg; ½ tsp mustard powder; small handful of thyme; 150g fresh or frozen peas; 150ml double cream; 500g block puff pastry; 1 egg, beaten; steamed green veg and new potatoes, to serve