Toss the cubed chicken with 1 teaspoon cornstarch and a pinch of salt. Let it sit while you prep everything else. The cornstarch keeps the lean breast meat from drying out.
Stir the sauce together in a small bowl: oyster sauce, soy sauce, fish sauce, sugar, water, and the remaining 1 teaspoon cornstarch.
Heat 1 teaspoon oil in a wok or large skillet over medium. Toast the cashews for 2 to 3 minutes until golden, then tip them onto a plate. Watch them, they burn fast.
Turn the heat to high and add the 1 tablespoon oil. Spread the chicken in one layer and let it sear undisturbed for 2 minutes, then stir fry until just cooked through, about 3 minutes total. Push it to the side.
Add the dried chilies and garlic to the open space and stir for 20 seconds until fragrant. Add the onion and bell pepper and stir fry 2 minutes, keeping the heat high so everything stays crisp.
Pour in the sauce and toss to coat. It will thicken in under a minute. Kill the heat, fold in the cashews and green onions, and serve over rice.