Preheat the oven to 250 degrees F.
Place the bread cubes on 2 baking sheets and bake, stirring halfway through, until the bread is completely firm and toasted, about 5-10 minutes.
In a large, deep, nonstick skillet over medium heat, add the butter and cook until it has melted.
Add the onions, 2 large stalks of the celery, the sage, 1/2 cup of the parsley, the turkey seasoning, 1/4 teaspoon of the salt, and 1/4 teaspoon of the pepper to the butter and sauté over medium-low heat until soft, about 5-10 minutes.
Transfer the veggie mixture from the heat and allow it to cool for a few minutes.
Transfer the veggie mixture to a large bowl and add the toasted bread.
In a small bowl, add the egg and the chicken broth and whisk to combine.
Add the egg mixture to the bread mixture and mix to combine.
Adjust the oven temperature to 425 degrees F.
Wipe the same skillet clean and add the oil over medium heat.
Add the shallots, the remaining parsley, the thyme, and the sage to the oil and cook until almost softened, about 2-3 minutes.
Add the remaining celery to the veggies and cook until they are all soft, about 3-5 minutes.
Add the mixed vegetables, the turkey, 2 cups of the turkey broth, the remaining salt, and the remaining pepper to the veggies and bring to a boil.
Bring the turkey mixture to a simmer over medium-low heat and cook, constantly stirring, until the flavors meld, about 10 minutes.
In a bowl, combine the remaining turkey broth with the cornstarch and mix well to dissolve.
Add the cornstarch mixture to the turkey mixture and cook, while stirring, until thickened, about 4-6 minutes.
Transfer the turkey mixture to a 9x13-inch baking dish.
Place the crust over the turkey mixture.
Bake until the crust is golden and the filling is hot and bubbling, about 30-45 minutes.
Serve.