Make sauce:Whisk together garlic, water, light soy sauce, dark sweet soy sauce, and oyster sauce. Whisk in cornstarch. Add red pepper flakes and white pepper (if using). Set aside. Prep ingredients:Slice onion and bell pepper and combine. Chop green onions. Dice chicken and toss with a pinch of salt and flour. Set onions / peppers, green onions, chicken, sauce, and cashews near the stove. Place a paper towel on top of a plate and set that near the stove.
Heat cooking oil in a wok over medium-high heat.
When oil is hot, add cashews and saute until golden brown and toasted, 1 to 2 minutes (watch carefully to prevent them from burning). Set cashews on paper-towel lined plate. Leave oil in the wok.
To remaining oil in wok, add chicken. Let chicken cook for 2 minutes without stirring (this will help it to brown on the bottom).
Continue cooking, turning occasionally, until chicken is golden brown and cooked through, 6 to 7 minutes total.
Transfer chicken to the plate with the cashews and return wok to heat.
To any remaining oil in the wok, add onions and bell peppers.
Saute until tender, 3 to 4 minutes. (Add a splash of water if the pan starts to look dry.)
Pour sauce over onions and peppers.
Simmer until sauce starts to thicken, 1 to 2 minutes.
Add green onions, chicken, and cashews to pan. Cook everything together for 1 minute to coat in sauce.
Serve over rice.