Heat a wok or large skillet over medium-high heat and add 2 tablespoons oil.
When the oil is hot, add the minced garlic, halved onion, and dried red chilies. Stir-fry until fragrant, about 30–60 seconds.
Add the roasted cashew nuts and stir briefly to coat and warm them.
Add the cubed chicken and stir-fry until the surface turns opaque, about 2–3 minutes.
Pour in the soy sauce, sweet soy sauce, fish sauce, sugar, and 2 tablespoons water. Stir to combine and continue stir-frying until the chicken is cooked through, about 2–3 more minutes.
Stir in the scallion pieces, toss to combine, then remove from heat and serve immediately.