1/2 cup unsalted stock of any kind or water
1 tablespoon sugar
1 tablespoon soy sauce
1 tablespoon Chinkiang black vinegar (or 1 1/2 teaspoons balsamic vinegar and 1 1/2 teaspoons rice vinegar)
1/2 teaspoon potato starch
1 pound long Chinese or Japanese eggplants (about 3 small)
Fine sea salt
Vegetable oil (for frying)
1/4 to 1/2 cup potato starch or cornstarch
1 1/2 tablespoons Sichuan chili bean paste or pickled chili paste, see Note
1 tablespoon finely chopped garlic
1 tablespoon finely chopped fresh ginger
2 green onions, (thinly sliced, white and green parts kept separate)
MAKE THE SAUCE: In a small bowl, whisk all the sauce ingredients until blended. Set aside.
Cut the eggplants lengthwise into 3-inch sections, then slice them into ½-inch wedges. In a large bowl, combine 3 cups water and 2 teaspoons fine sea salt and whisk until the salt dissolves, then submerge the eggplant and soak for 15 minutes. Drain and pat the wedges dry. Salting helps relax the flesh, reduce any bitterness, and prevent it from soaking up excessive oil.
Heat about 1 inch of oil (roughly 1 1/2 cups) in a wok or skillet over medium-high heat. Coat the eggplant lightly in the starch. When the oil reaches 375°F, fry the eggplant in batches, flipping and turning it occasionally to cook evenly, until the edges are slightly golden and the skin is glossy purple and wrinkled, 3 to 4 minutes. Transfer the cooked eggplant to a paper towel–lined plate. Pour the oil through a paper towel lined strainer set over a bowl and reserve for another use. Wipe the wok clean then return 1 tablespoon oil to the wok.
Return the wok to the stove over medium heat and add the chili bean paste. Stir-fry over low heat until its red oil is released, about 30 seconds. Add the garlic, ginger, and scallion whites and stir-fry until just aromatic, about 30 seconds more. Push the aromatics up one side of the wok and pour the sauce mixture into the center.
Gently fold the fried eggplant into the sauce and simmer for about 2 minutes, until the eggplant has absorbed the flavors and the liquid is thickened from the starch. Transfer to a plate, garnish with the scallion greens, and serve immediately.