Add water and mirin or sake and cover and simmer until tender.
Mix miso, soy sauce and sugar in a small bowl.
Add to the eggplant mixture and stir-fry.
Optional: For a thicker "gravy", Mix optional cornstarch and water together. Bring to boil and add cornstarch mixture. Let boil for one more minute, stirring constantly and lower temperature to simmer for 5 more minutes.