Mix the soy sauce and brown sugar together until the sugar dissolves, add the chicken strips, toss to coat evenly, and refrigerate for 30 minutes.
Heat 3 tablespoons oil in a wok over medium-high heat, add the marinated chicken, and stir-fry for 2 to 3 minutes until the internal temperature reaches 74°C (165°F).
Add the onion quarters and crushed garlic, stir-fry for 2 minutes.
Add the bean sprouts and cook for 4 to 5 minutes.
Stir in the dark sesame oil.
Mix the cornstarch with water until smooth and lump-free, pour into the wok with the chicken broth while stirring constantly, bring to a boil, and continue stirring for 1 to 2 minutes until the sauce thickens.
Transfer to a serving bowl, garnish with the leek strips, and serve.