Combine all the sauce ingredients: In a small bowl, whisk together the soy sauce, rice vinegar, hoisin sauce, brown sugar, sesame oil, and sriracha until smooth. Set this aside as it will be the flavor powerhouse of your dish.
Prepare the chicken: Cut your chicken breast into bite-size pieces. In a zip-close bag, season the pieces with salt and black pepper, then toss them with cornstarch until they are evenly coated. This step is crucial for that lovely crispy texture.
Heat the oil: In a large wok or skillet, heat 1 tablespoon of olive oil over medium-high heat. Ensure the oil is hot but not smoking; this is key to getting those vegetables to char beautifully.
Add the vegetables: Once the oil is hot, add the sliced onion, zucchini, and mushrooms. Stir-fry them for about 3 to 5 minutes or until they are lightly charred and tender-crisp. Then, remove them from the wok into a bowl and set aside.
Cook the chicken: In the same wok, add the remaining olive oil and heat it up. Add the coated chicken pieces in a single layer. Allow them to cook without stirring for a few minutes until they turn golden brown and crispy. Depending on the size of your pan, you may need to do this in batches to avoid overcrowding.
Combine everything: Once the chicken is cooked through, pour in the prepared sauce. Cook for a couple of minutes until the sauce thickens slightly, then add the cooked vegetables and cashews. Stir everything together, making sure the sauce evenly coats all the ingredients.
Serve and enjoy: Serve your Cashew Chicken Stir Fry over rice if desired. Enjoy this vibrant and flavorful dish with your loved ones!