Cook eggplant in 2-3 tbsp oil in a large deep skillet until tender (add a little more oil or water if necessary- it will cook down with the amount of oil stated but with a nicely hot wok, there may be a little smoke involved, but don't worry, turn on the hood exhaust because it just adds to the flavor).
While eggplant is cooking, mix soy sauce, sugar, and broth together in a small bowl till sugar dissolves.
Remove eggplant from skillet when cooked and keep warm.
Heat garlic, red pepper flakes, and ginger in 1 tbsp oil until aromatic.
Stir in the water chestnuts and half of the scallions along with the eggplant and the soy sauce mixture.
Stir and cook until the soy sauce is absorbed into the eggplant (this may take several minutes).
Add the vinegar, sesame oil, and remaining scallions, stirring to mix.