Preheat oven to 400 degrees F. Lightly coat a 2-quart rectangular baking dish with cooking spray. Set aside.
For sauce, in a medium bowl whisk together broth, hoisin sauce, ginger, cornstarch, crushed red pepper and black pepper; set aside.
Lightly coat an extra-large skillet with cooking spray or olive oil; heat over medium-high heat. Add chicken to skillet; cook until lightly browned. Remove from skillet. Add onion wedges, bok choy, celery, and sweet pepper to the skillet.
Cook for 3 to 4 minutes or until vegetables start to soften. Add garlic; cook for 30 seconds more. Add in drained can of sliced water chestnuts. Stir in the sauce.
Cook and stir about 3 minutes or until sauce is thickened and bubbly. Stir in cooked rice and browned chicken.
Spoon chicken-rice mixture into the prepared baking dish. Cover with foil and bake about 20 minutes or until casserole is bubbly and chicken is no longer pink (165 degrees F).
Sprinkle chow-mein noodles, cashews and hemp hearts over top. Bake, uncovered, for 4 to 5 minutes more or until noodles and cashews are golden brown. Sprinkle with green onions.