Combine turkey, cream cheese, stuffing, 1/4 cup gravy, peas, and onion in large microwave-safe bowl. Heat in the microwave until cream cheese softens, about 2 minutes.
Spread mashed potatoes over the bottom of a 9x11-inch casserole dish.
Cut crescent roll dough in half. Flatten both halves on a work surface. Spoon turkey mixture down the middle. Roll up edges and press to seal. Place over potatoes.
Combine remaining 1/4 cup gravy, cream of chicken soup, and milk in a blender; blend until smooth. Pour over stuffed dough.
Bake in the preheated oven until bubbly and hot, about 30 minutes.