Take buttermilk, out of refrigerator to warm to room temperature. Wash green beans and cut into bite sized pieces. Steam until tender. While the beans cook, chop the onion and sauté in oil or butter. Slice mushrooms in thick pieces and add them to the onion. Cook and stir until tender. Cube eggplant and add to mushrooms. Cover and steam until tender, stirring as necessary to prevent sticking. Add green beans and season with salt and pepper. Beat buttermilk smooth. Stir cornstarch into the beaten buttermilk and bring it to a boil, stirring vigorously; it will separate and then smooth out again. Add to the bean mixture. Serve warm