Place a ramekin or biscuit cutter in the center of the skillet. Set the pie plate with the tofu on the ramekin, cover and steam until the tofu is hot, about 10 minutes. Arrange the tofu around the spinach, cover and keep warm. Pour the tofu sauce into a saucepan and bring to a simmer. Stir in the cornstarch mixture and cook over moderate heat, stirring occasionally until thickened, about 3 minutes. Pour the sauce over the tofu and serve.