Make the sauce: in a small bowl or glass measuring cup, use a fork to whisk the stock with 1 tbsp of the cornstarch until no lumps remain. Whisk in the soy and hoisin sauces, then the garlic. Set aside.
In a medium bowl, toss chicken pieces with remaining 2 tablespoons of cornstarch.
Heat the oil in a large skillet or a wok over medium-high heat. Add half of the chicken and cook in a single layer until golden on both sides and no longer pink inside. Transfer to a clean bowl, and repeat with remaining chicken, placing second batch in the bowl when cooked through.
Add the bok choy and white part of green onions to the same skillet; cook until the stem of the bok choy is crisp-tender, about 3 minutes. Return cooked chicken to skillet along with the sauce, the green onion tops and the cashews. Cook until sauce thickens a bit, and everything is heated through, about 2 minutes. Serve with rice or another grain (I used a quinoa/amaranth combination).