Heat oven to 350 degrees.
In a large skillet over medium-high heat, melt butter. When the butter starts to foam add the leeks and carrot, cook and stir with a wooden spoon or spatula, until the carrots are softened (or to preferred doneness), 5 to 7 minutes. Add in the shallots, parsley, thyme, garlic and onion powder, cook and stir for another minute.
Add in the flour, cook and stir until slightly browned, 2 minutes. Add the chicken stock and milk and bring to a simmer, cook and stir until sauce is thickened, about 3 to 5 minutes. Sauce is ready when it easily coats a spoon. Turn mixture into prepared pan and set aside.
In a small bowl, mix together egg and heavy cream. Brush top edge of dish with egg wash. Place puff pastry on top and trim it as needed so that the puff pastry fits just to the edge of the dish. Use a fork and lightly crimp the puff pastry against the edge of the dish.
Transfer dish to the oven and bake at 350F for about 35 minutes or until top of puff pastry is a dark brown. Remove from oven and serve immediately.
