Whisk together soy sauce, fish sauce, sugar, and water until the sugar dissolves; set the sauce aside.
Heat a large skillet or wok over high heat and add the oil.
Add the sliced chicken to the hot skillet and cook until lightly browned on both sides, about 2–3 minutes per side.
Push the chicken to one side of the skillet to make room for aromatics.
Add the minced garlic and chopped onion to the empty side and sauté for about 30 seconds until fragrant.
Add the red and green bell peppers and the cashews; toss and stir-fry with the chicken for 1–2 minutes.
Pour the prepared sauce over the chicken and vegetables, stir to coat, and cook an additional 1–2 minutes until the sauce slightly thickens.
Stir in the scallion strips, remove from heat, and serve immediately with steamed rice if desired.