In a large mixing bowl, add the chicken, salt, pepper, and arrowroot. Toss until evenly coated.
Heat the oil in a large skillet set over medium-high heat. Once hot, add the chicken, working in batches as needed, and cook on both sides until golden brown, 3 to 4 minutes per side. Transfer to a plate and repeat with the remaining chicken.
Add the onions and cook, stirring, until softened, about 3 minutes. If your pan is dry, add an additional 1 tablespoon oil.
Add the bok choy, garlic, and water chestnuts. Cook until the bok choy is tender, about 5 minutes.
Reduce the heat to medium. Add the ginger, tamari, rice vinegar, and crushed red pepper flakes (if using). Stir to combine. Let simmer until the sauce thickens, 3 to 5 more minutes.
Reduce the heat to low and add the chicken back into the skillet. Gently toss until the chicken is well-coated in the sauce and reheated, 1 to 2 more minutes. Taste and add additional salt and pepper as needed.
Serve as desired and enjoy!