Chicken, kale & mushroom pot pie

Name: Chicken, kale & mushroom pot pie
Prep time: 10 min
Cook time: 40 min
Total time: 50 min
Cuisine: European, British
Ingredients: 1 tbsp olive oil; 1 large onion, finely chopped; 3 thyme sprigs, leaves picked; 2 garlic cloves, crushed; 350g chicken breasts, cut into small chunks; 250g chestnut mushrooms, sliced; 300ml chicken stock; 100g crème fraîche; 1 tbsp wholegrain mustard; 100g kale; 2 tsp cornflour, mixed with 1 tbsp cold water; 375g pack puff pastry, rolled into a circle slightly bigger than your dish; 1 egg yolk, to glaze