Combine chicken, honey, soy sauce, rice vinegar, sambal oelek, ginger, and sesame oil in a large resealable plastic bag. Seal and place in the refrigerator to marinate, about 2 1/2 hours.
Bring water to a boil in a small saucepan. Stir in rice. Reduce heat and simmer, covered, until rice is tender, about 15 minutes. Sprinkle sesame seeds over the surface of the rice. Continue cooking until water is completely absorbed, about 5 minutes more. Fluff rice with a fork to fully incorporate sesame seeds.
Heat vegetable oil in a large skillet or wok over high heat. Cook and stir onion until browned, 5 to 10 minutes. Add chicken mixture, including marinade, and garlic; saute until chicken is no longer pink in the center and sauce coats chicken pieces, 5 to 8 minutes.
Stir mangoes and cashews into the skillet until just warmed through, about 2 minutes. Serve over rice. Garnish with black pepper.