In a medium bowl, toss chicken with cornstarch until chicken is coated. Season with salt and pepper. In a large skillet, heat the oil over medium-high heat. Cook the chicken, tossing often, for 3-5 minutes. Add garlic and white parts of scallions to the skillet. Continue cooking until chicken is browned, about 3 minutes more. Add vinegar and cook until evaporated, about 30 seconds. Stir in hoisin sauce and 1/4 cup water. Cook and stir until chicken is cooked through. Remove from heat. Stir in scallion greens and toasted cashews. Serve immediately over white rice, if desired.