Combine soy sauce, vinegar, sherry, sugar in a liquid-measuring cup. Add enough water to make up to 1 cup. Place cornstarch in a small bowl, pour in the liquid, pour on the liquid & whisk till dissolved. Set aside.
Heat a large wok over a high flame. Add oil & onion & stir fry for about a minute. Add eggplant & salt & stir fry for 8 to 10 minutes till the eggplant is soft. Add garlic, ginger & black pepper & cayenne. Cook a few minutes more.
Add tofu & scallion bottoms. Stir the bowl of liquid that has been set aside & add to the wok. Mix well & stir fry for another few minutes till the sauce is thickened.
Remove from the heat & serve over rice topped with scallion greens & cilantro.