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.