Begin by seasoning the chicken pieces with salt and freshly ground black pepper.
In a small bowl, whisk together the hoisin sauce, water, soy sauce, cornstarch, rice vinegar, and sesame oil.
Heat the vegetable oil in a wok or large skillet over medium-high heat. Once shimmering, add the seasoned chicken pieces. Cook for about 5-7 minutes until golden brown and cooked through.
In the same pan, add the minced garlic and the white parts of the scallions. Stir-fry for about 30 seconds until fragrant.
Return the chicken to the pan and pour the sauce over it. Stir to coat evenly and simmer for 2-3 minutes until the sauce thickens.
Fold in the roasted cashews and the green parts of the scallions. Toss gently to mix.
Serve hot over steamed rice or noodles.