Whisk together the soy sauce, fish sauce, sugar, and water until the sugar is fully dissolved; set the sauce aside.
Heat a large skillet or wok over high heat and add the oil.
Add the chicken pieces and cook, stirring occasionally, until lightly browned on both sides and nearly cooked through.
Push the chicken to one side of the skillet. In the open space, add the minced garlic and onion and sauté about 30 seconds until fragrant.
Add the red and green bell peppers and cashews, toss to combine, and stir-fry briefly with the chicken.
Pour the prepared sauce over the skillet contents, stir to coat everything evenly, then add the scallion strips and cook another 30–60 seconds until the sauce thickens slightly and the scallions are wilted.
Turn off the heat and serve immediately with steamed rice if desired.
