Melt 1 tablespoon unsalted butter with 1 tablespoon extra-virgin olive oil in heavy large skillet over medium-high heat. Add chopped onion and sauté until golden, 7 to 8 minutes. Add sausage; stir to combine. Add sliced mushrooms; sprinkle with salt and pepper. Sauté until browned and tender, about 4 minutes. Add mixed vegetables; cover and cook until carrots soften slightly, about 2 minutes. Add leftover vegetables, if desired. Add diced turkey, gravy, and chopped fresh thyme. Bring filling mixture to boil; reduce heat to low and simmer to thicken if necessary. Season to taste with salt and pepper. Mix in peas and chopped parsley. Pour filling into 9-inch-diameter glass pie dish and refrigerate until cool, about 30 minutes.

Roll out pie crusts and cut slightly larger than individual baking dishes. Cut pie crusts slightly larger than baking dishes. Divide filling among 6 individual baking dishes. 

Lightly brush rim of filled dishes with egg wash. Place crust atop each individual dish. Pinch dough rim to seal crust all around. Preheat oven to 400°F. Lightly brush top of pies with egg white to glaze. Make three to four 1/4-inch slits in center of pies to allow steam to escape. Bake pies until crust is golden and filling is bubbling, 30 to 35 minutes. Remove from oven and let cool 10 minutes.

Note: to freeze, prepare filling for turkey pot pie; freeze for up to two months. Thaw and proceed with recipe.