In a medium bowl, toss 1 1/2 pounds boneless, skinless chicken breasts, with 1 tablespoon cornstarch until chicken is coated. Season with salt and pepper.
In a large skillet, heat the 2 tablespoons vegetable oil over medium-high heat. Cook the chicken, tossing often, for 3-5 minutes.
Add 6 cloves garlic and white parts of 8 scallions to the skillet. Continue cooking until chicken is browned, about 3 minutes more.
Add 2 tablespoons rice vinegar and cook until evaporated, about 30 seconds.
Stir in 3 tablespoons hoisin sauce and 1/4 cup water.
Cook and stir until chicken is cooked through. Remove from heat.
Stir in scallion greens and 3/4 cup raw cashews. Serve immediately over white rice, if desired.