Preheat the oven to 400 degrees. Using a large sized saucepan, bring the vegetable broth to a boil. Add the sweet potatoes, carrots, onions, mushrooms and leeks. Season with salt and pepper and let simmer for about 10 to 15 minutes, or until the vegetables are tender. Add the kale in for the last 5 minutes only.
Meanwhile, using a small bowl, whisk together the milk and flour until there are no lumps. After the vegetables are tender, slowly pour in the flour mixture to the pan, stirring until everything thickens.
Season with a little more salt and pepper, remove from the heat and mix in the Dijon mustard and mascarpone. Taste and adjust any ingredients as needed.
Lay one defrosted puff pastry sheet into a casserole dish, adjusting to fit the dish. Pour the vegetable filling into the dish, onto the puff pastry sheet and cover with the second puff pastry sheet, trimming off any excess and piercing the puff pastry dough on top to allow steam to escape while baking.
Using a pastry brush, brush on the egg wash over the top of the pot pie and bake for about 25 to 30 minutes, or until golden brown and heated through.