Heat oven to 375°F. Spray 3-quart casserole with cooking spray. In large bowl, mix turkey, potatoes, frozen vegetables, mushrooms, sour cream, gravy and sage; spoon mixture into casserole.
Unroll pie crust on work surface. Cut into 1 1/2-inch wide strips, then cut in opposite direction, making 1 1/2-inch square pieces (not all will be perfectly square). Starting with rounded-edge pieces around edge of casserole, cover top of mixture with pie crust pieces, overlapping each piece (see photo).
Bake 1 hour 15 minutes to 1 hour 20 minutes or until crust is golden brown and edges are bubbly.