Preheat the oven to 400F/200C. Have a baking dish around 10in x 7in (25cm x 18cm) and at least 1 ½in (4cm) deep ready - you can also use other shapes but with roughly the same volume and similar size on top to keep a similar ratio of crust to filling (if anything, slightly smaller and deeper is better but not wider as the filling will be too shallow).
Trim and dice the chicken, if not already cut up, into roughly bite sized pieces. Sprinkle with salt and pepper all over. Trim the ends off the leeks then cut them into medium slices, cutting any large slices in half. Finely chop the garlic.
Warm roughly half of the butter in a medium-large skillet/frying pan over a medium-high heat and add the chicken - most likely, you are best to add in two batches so the pan isn't overcrowded. You want to sear the chicken all over to get a nice gently brown sear. Remove the chicken from the pan once seared and set aside, and cook next batch and set that aside as well.
Reduce the heat to medium-low and add the rest of the butter then, once melted, the leeks. Cook the leeks, stirring now and then, until it becomes nicely soft. Take care as leeks can easily brown if your heat is too high or you don't have enough fat - they are more delicate to cook than onions. So reduce heat or add additional butter if needed.
Once the leeks are soft, add the garlic, cook briefly, then deglaze with wine to get all the flavor from the pan into the sauce. Sprinkle in the flour to absorb the excess fat and stir to mix in smoothly. Add the stock, mustard and tarragon (or other herb, if you prefer). Add back the chicken, mix through and cook briefly. Stir in the sour cream off the heat and set aside.
Place a baking dish upside down on the puff pastry and cut a piece just slightly larger than the dish - I ended up using around ⅔ of the sheet.
Fill the baking dish with the filling mixture then top with the piece of pastry. Fold in the edges to give a double layer around the edge but still with the pastry going all the way to the edge of the dish. Press the edges down with a fork to give a pattern and seal it against the dish.
Cut a couple slits in the middle of the pastry to give ventilation for steam as it cooks. Whisk the egg and brush the top with the eggwash to help the pastry to brown as it bakes.
Place in the oven and bake for approximately 30 minutes until the pastry is golden brown and crisp. Let it rest a couple minutes before cutting open and serving, getting a good balance of pastry and filling.