Heat oven to 375°F. In 10-inch skillet, heat oil over medium-high heat. Add onion and celery. Cook 3 to 5 minutes, stirring occasionally, until crisp-tender. Stir in carrots, green beans, chicken, soup, gravy and sage. Cook until bubbly. Stir in 3/4 cup of the cheese. Spoon into ungreased deep 2-quart casserole.
Place pie crust over hot chicken mixture. Fold over edges to fit inside casserole. Cut small slits in surface of crust with paring knife.
Bake 40 minutes. Sprinkle remaining 1/4 cup cheese over crust; bake 5 to 6 minutes longer or until crust is deep golden brown and cheese is melted.