Heat the peanut oil in a wok or heavy bottomed skillet over medium-high heat. Fry the cashew nuts until they just start to change color. Remove from pan and set on paper towel.
Add the garlic and let cook for 30 seconds. Add the sliced chicken along with the fish sauce, brown sugar, and chili jam. Stir-fry for 1 minute, then add the bell pepper, scallions, and onion. Continue to stir-fry until vegetables are tender-crisp, then add the mango and return the cashews to the pan. Toss well, and remove from heat.