To prepare the chicken, cut the chicken thighs into small pieces and place them in a mixing bowl. Season first with salt and mix until absorbed. Add the white pepper, soy sauce, and dark soy sauce and mix well. In a small bowl, combine the cornstarch and water to make a slurry, then add it to the chicken and mix until evenly coated. Add the cooking oil and mix again to seal the marinade. Let the chicken marinate while preparing the remaining ingredients.
To prepare the vegetables, slice the mushrooms, cube the onion, slice the scallions, and mince the garlic and ginger.
To make the sauce, in a bowl, whisk together the soy sauce, dark soy sauce, oyster sauce, chicken stock, and cornstarch mixed with water until smooth.
To stir-fry the chicken, in a wok over high heat, add 1 tablespoon of cooking oil. Add the marinated chicken and stir-fry for about 3 minutes until fully cooked through. Remove and set aside.
To stir-fry the aromatics and mushrooms, in the same wok over high heat, add the remaining tablespoon of cooking oil. Add the onion, mushrooms, garlic, and ginger and stir-fry briefly until fragrant and slightly softened.
To combine everything, return the chicken to the wok and pour in the sauce. Stir-fry until the sauce thickens and evenly coats the chicken and mushrooms. Lastly, add the scallions and give everything a quick toss before serving.