Start by slicing the chicken into thin strips. Mix soy sauce, rice vinegar, cornstarch, sesame oil, and black pepper in a bowl, then add the chicken. Stir to coat, and set it aside for 15 minutes. This marinade infuses the chicken with flavor and keeps it tender as it cooks.
Bring a pot of water to a boil, then blanch the broccoli florets for about 2-3 minutes. This process softens the broccoli slightly and helps them retain their beautiful green color. Drain them and set them aside.
Heat some vegetable oil in a large skillet or wok over medium-high heat. Add the marinated chicken, spreading it out so it cooks evenly. Sear each side for 2-3 minutes until golden brown and fully cooked. Remove the cooked chicken and set it aside.
Using the same skillet, sauté minced garlic and grated ginger until fragrant. Combine chicken broth, oyster sauce, soy sauce, and a little cornstarch in a separate bowl, then pour it into the skillet. Reduce the heat and whisk the mixture until it thickens. This savory sauce ties everything together.
Return the chicken to the skillet, along with the blanched broccoli. Toss everything until the sauce coats each piece evenly and the dish is warmed through. Adjust seasoning if needed, and serve it hot over steamed rice!