1Prepare all ingredients before you start cooking as stir-frying happens very quickly. Cut the chicken into bite-sized pieces, about 1-inch cubes. Place the chicken pieces in a bowl and toss with the cornstarch until lightly coated - this helps create a silky texture and helps the sauce cling to the meat. Cut the bell pepper and onion into chunks similar in size to the chicken pieces for even cooking. Cut the green onions into 2-inch pieces, separating the white parts from the green tops. Mince the garlic. If your cashews are raw, toast them in a dry pan over medium heat for 3-4 minutes, stirring constantly, until golden and fragrant. Set aside.
2Prepare the sauce by combining the oyster sauce, soy sauce, fish sauce, dark soy sauce, sugar, and chicken broth in a small bowl. Stir until the sugar dissolves. The dark soy sauce adds color and a slight molasses-like sweetness - if you do not have it, you can use all regular soy sauce but add a tiny pinch more sugar. Taste the sauce - it should be savory with a balance of salty, slightly sweet, and umami. Set aside within easy reach of your cooking station.
3Heat a wok or large skillet over high heat until smoking. Add 2 tablespoons of the vegetable oil and swirl to coat. Add the dried red chilies and stir-fry for about 20-30 seconds until they darken slightly and become fragrant - be careful not to burn them or they will turn bitter. The chilies will release their aroma and add a subtle smoky heat to the oil. Remove the chilies and set aside. Add the cornstarch-coated chicken pieces to the hot wok in a single layer. Let them sear without stirring for about 45 seconds to develop a light golden crust, then stir-fry for another 2-3 minutes until the chicken is almost cooked through but still slightly pink inside. Transfer the chicken to a plate.
4Add the remaining tablespoon of oil to the wok. Add the minced garlic and white parts of the green onions. Stir-fry for 30 seconds until fragrant. Add the bell pepper and onion chunks. Stir-fry over high heat for 2-3 minutes until the vegetables are crisp-tender - they should have some charred spots but still retain their crunch. The high heat is essential for achieving that characteristic wok hei flavor and keeping the vegetables vibrant rather than soggy.
5Return the chicken to the wok along with any accumulated juices. Give the sauce a quick stir and pour it over the chicken and vegetables. Toss everything together vigorously, stirring and flipping constantly for about 1 minute. The sauce should coat all the ingredients evenly and become slightly glossy as it reduces. Add the toasted cashews and the reserved fried dried chilies. Toss for another 30 seconds to warm the cashews and distribute them throughout the dish. The sauce should cling to the ingredients without pooling at the bottom of the wok.
6Remove the wok from heat and add the green onion tops. Toss briefly - the residual heat will wilt them slightly while preserving their fresh color and flavor. Taste and adjust seasoning if needed - add a splash more fish sauce for saltiness, a pinch of sugar for sweetness, or a squeeze of lime juice for brightness. Transfer to a serving plate immediately. The dish should be glossy and aromatic, with the cashews prominently visible throughout. Serve hot with steamed jasmine rice. The dried chilies are typically left in for presentation and a bit of extra heat for adventurous diners, though they are quite intense to eat whole. Cashew chicken is best enjoyed immediately while the cashews are still crunchy and the vegetables are crisp-tender.