Chicken and leek casserole with flaky pastry

Name: Chicken and leek casserole with flaky pastry
Prep time: 15 min
Cook time: 50 min
Total time: 1 h 5 min
Cuisine: European, British
Ingredients: 30g butter; 1 leek, sliced; 850g chicken thigh; 250.00 ml frozen peas and corn; 1 bunch asparagus, roughly chopped; 490g jar Golden Honey & Mustard Chicken Tonight; 62.50 ml chopped fresh oregano; 1 egg, lightly beaten; 1 sheet frozen puff pastry, thawed