1. Cook pasta according to package directions.
2. Meanwhile, in a large skillet, brown chicken in 1 tablespoon oil. Remove and set aside. In the same pan, saute mushrooms in remaining oil until tender; set aside. In a small bowl, combine the egg, ricotta, 1-1/2 cups Parmesan, mozzarella and seasonings.
3. Drain and rinse pasta with cold water; stuff each shell with about 1 tablespoon of cheese mixture. Place in a greased 13-in. x 9-in. baking dish. Top with chicken and mushrooms.
4. In a large saucepan over medium heat, melt butter. Add garlic; cook and stir for 1 minute. Add cream; cook 5 minutes longer. Add 1-1/2 cups Parmesan cheese; cook and stir until thickened.
5. Pour sauce over the casserole. Sprinkle with remaining Parmesan cheese. Cover and bake at 350° for 30 minutes. Uncover; bake 10-15 minutes longer or until bubbly.
6. If desired, cover and freeze unbaked shells for up to 1 month.
7. To use frozen shells: Thaw in the refrigerator. Let stand at room temperature for 30 minutes. Bake as directed. Yield: 10 servings.