In a bowl, combine the cubed chicken with the marinade of soy sauce, cornstarch, and rice wine. Set aside to marinate for 15-20 minutes.
In a small bowl, stir together the Sichuan sauce ingredients - soy sauce, dark soy sauce, rice vinegar, Spicy Sichuan Seasoning, Chinese rice wine, sugar, cornstarch and water until combined. Set aside.
Heat 2 tablespoons of oil in a wok or frying pan over high heat. When the oil is hot, add the chicken pieces, spreading it out as much as possible.
Sear for 1 minute without stirring until the edges are lightly browned, then stir to cook all sides until just cooked through - about 2-3 minutes more. Transfer chicken to a bowl and set aside.
Swirl in another tablespoon of oil into the same wok. Reheat, then add in garlic.
Stir-fry over low heat for about 10-15 seconds, or until aromatic, taking care not to burn the garlic.
Turn heat up to high, add in mushrooms and cook for about 2 minutes.
Next, add in the snow peas and bell peppers, and stir-fry for another 30 seconds. If the vegetables get a bit dry, add in one or two splashes of water as you cook.
Add in red onion and cook for another minute – making sure to stir and toss the ingredients well.
When the vegetables are bright and crisp-tender, stir in the cooked chicken.
Then take the prepared sauce, give it a stir, and quickly pour it into the wok. Cook until the sauce thickens.
Add in one or two tablespoons of water if needed. Toss well, and cook for a further 30 seconds or until the mixture is hot and bubbly.
Finally, add in the chopped cashews. Turn heat off, give everything a final stir and serve.