Heat oil in a large skillet add the garlic, and cook over medium until light brown and fragrant, about 30 seconds. Increase heat to medium high, and add chicken; cook, stirring often, until just seared but not yet cooked through, about 2 minutes
Add salt and pepper; stir in onions and bell peppers; cook, stirring often, until vegetables are light brown and soft, 1 to 2 minutes. Add cashews and chilis; stir-fry until heated through, about 1 minute.
Add oyster sauce and black soy sauce; cook, stirring constantly, until the flavors meld and sauce reduces to a glaze, 1 to 2 minutes. Adjust seasonings to taste; serve over hot rice.