How can I make the crust of my chicken pot pie crunchy?
Just a thought, you might like to swap your normal breadcrumbs for "Panko" Japanese crumbs, (available in most supermarkets)
these crisp up more efficiently with tiny dots of butter on top. The other trick I often do for crispy breadcrumb toppings, is to add a small amount of dry couscous to the mix, before baking.

I also roll wet potatoes in dry couscous, to make roast potatoes really crispy, The extra texture and crunch is great.