Pre-heat your oven to 200C (405F). Slice the leeks, white and light green parts only. Chop the mushrooms and cut the chicken into bite-size pieces.
Heat butter in a skillet and add leeks, mushrooms, garlic, and Italian herbs. Cook them for 2-3 minutes.
Add the chicken and cook until it is no longer pink, for 5-6 minutes.
Now add whole-grain mustard and cooking cream. Season to taste with salt and pepper.
Transfer the leek filling into the pie dish.
Top with the rolled out puff pastry. Trim the excess pastry and make a few slits on the surface of the pastry. This will allow the steam to escape. Bake the pie for 30 minutes.
Notes
Step-by-Step Photos: My blog post includes helpful step-by-step photos to help guide you through making this recipe.Substitutions & Variations: I have included a list of these in the post above.To Store: Store in an airtight container in the fridge for 2-3 days. Reheat in the microwave.To Freeze: This is best frozen before cooking. Cover in cling film and freeze for up to two months. Cook from frozen or thaw first.Tip 1: The chicken doesn't need to be completely cooked through in the pan as it will continue to cook in the oven.Tip 2: If you want the sauce a bit thinner, you can use chicken stock or broth. Tip 3: You can make individual Chicken and Leek Pies by pouring the filling evenly into small ovenproof containers and covering with just enough puff pastry.