First things first, let's make our super simple garlic sauce. In a small bowl, whisk together the soy sauce, rice vinegar, sugar, cornstarch, and water until everything is combined and the cornstarch is dissolved. Set this aside – it's your flavor powerhouse!
Now, heat that vegetable oil in a large pan or wok over medium-high heat. Once the oil is shimmering, add your cut eggplant pieces. Stir-fry the eggplant for about 5-7 minutes, stirring often, until it starts to soften and turn golden in spots. The eggplant really soaks up the oil here, which is okay – it helps it get that lovely tender texture.
Next, push the eggplant to the side of the pan or just make a little space in the center. Add the minced garlic, ginger, and the white and light green parts of the chopped green onions to the hot spot. Sauté for about 30 seconds to a minute, just until they're fragrant – be careful not to burn the garlic!
Now, pour your whisked sauce mixture directly into the pan with the eggplant. Stir everything together gently but thoroughly, coating all the eggplant pieces. The sauce will thicken up pretty quickly thanks to the cornstarch. Let it bubble and simmer for another 1-2 minutes, just until the sauce is glossy and coats the eggplant beautifully.
Take the pan off the heat. If you're using it, stir in the optional sesame oil now. Taste and add red pepper flakes if you want some heat.
To serve, simply spoon this glorious garlic eggplant over hot, cooked rice. Garnish with the reserved green onion tops for a pop of color and freshness. Enjoy immediately!