In a large bowl or re-sealable plastic bag, mix together 2 tablespoons of the oil, the soy sauce, and the honey until well combined. Add chicken and toss to coat well. Place in refrigerator to marinate at least 30 minutes and up to 2 hours.
Meanwhile, heat a large frying pan over medium heat and add nuts. Toast nuts, shaking pan a few times, until nice toasted, about 2 minutes. Set aside to cool. (Can be made up to 3 days in advance.)
When chicken is ready, heat remaining 1 tablespoon oil in a large frying pan over medium heat. Add garlic and onion and cook until soft, about 2 minutes. Add chicken and marinade juices and cook, stirring occasionally, until chicken is browned on all sides and cooked through, about 6 minutes. Stir in mango and cashews and cook until heated through, about 1 minute. Taste and adjust seasoning, as necessary. Serve immediately over Sesame Rice.
Toasted Sesame Rice:
Bring water to a boil. Add rice and salt and stir to combine. Reduce heat to low, cover, and simmer until rice is tender and water is absorbed, about 10 minutes. Remove from heat and let stand, covered, to finish cooking, about 5 minutes. Stir in oil and sesame seeds and serve.