In a bowl, combine the vinegar, brown sugar, oyster sauce, sesame oil, 1/4 cup of the soy sauce and 1 tablespoon of the ginger. Set aside.
Season the chicken with salt and pepper. Heat the vegetable oil in a large skillet over medium-high heat. Add the chicken and saute until cooked through, 5 to 7 minutes.
In the same skillet, stir-fry the garlic, bell peppers and remaining 2 tablespoons ginger, keeping them crisp-tender. Deglaze the skillet with the sherry and scrape up any brown bits. Add the reserved soy mixture and cook until the mixture boils, about 1 minute. Whisk together the cornstarch and remaining 1/4 cup soy in a small bowl, then stir the slurry into the skillet. Add the cashews and water chestnuts. Add 1/4 cup water to thin the sauce. Cook for another minute.
Serve garnished with the scallions and cilantro.
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