Drain the mock duck in a colander and cut any large chunks in half to make bite-size pieces.
Premix the sauce by combining the sugar, Thai black soy sauce, Thai thin soy sauce, oyster sauce, and fish sauce to a small bowl. Set aside.
Heat a wok or skillet over high heat. When it’s lightly smoking, reduce the heat to medium high. Add the neutral oil and swirl to coat.
Add the garlic and cook for about 20 seconds. Then add the onions, mushrooms, and green beans. Stir-fry for 3-4 minutes. Add the red bell pepper and the premixed sauce. Cook, stirring, for another 2 minutes.
Add the mock duck and Thai basil. Stir-fry for another 2 minutes to heat the mock duck through, wilt the Thai basil, and coat everything evenly in the sauce. Serve immediately with steamed jasmine rice!