Recipe for Chinese-style Beef and Onion Stir-Fry:

1. **Prepare the ingredients:** Slice the beef thinly. Mince the garlic and slice the onions, bell pepper, carrot, mushrooms (if using), and scallions (separate the white and green parts).

2. **Make the sauce:** In a bowl, mix together soy sauce, oyster sauce (or hoisin sauce), rice vinegar, sugar, cornstarch, and beef broth (or water). Stir until the cornstarch is dissolved. Set aside.

3. **Cook the beef:** Heat a bit of vegetable oil in a wok or large skillet over high heat. Add minced garlic and the white parts of the scallions, stir-fry for a few seconds until fragrant. Add the sliced beef and stir-fry until browned and almost cooked through. Remove the beef from the pan and set aside.

4. **Stir-fry the vegetables:** In the same pan, add a little more oil if needed. Stir-fry the sliced onions (and other vegetables if using) until they are tender but still crisp.

5. **Combine with sauce:** Return the beef to the pan with the cooked vegetables. Pour the sauce mixture over the beef and vegetables. Stir constantly until the sauce thickens and coats the beef and vegetables evenly.

6. **Finish and serve:** Season with ground black pepper to taste. Garnish with the green parts of the scallions. Serve hot with steamed rice.

### Tips:
– For extra flavor, you can add freshly grated ginger along with the garlic at the beginning of cooking.
– Adjust the thickness of the sauce by adding more or less cornstarch mixture according to your preference.

This Chinese Beef and Onion Stir-Fry recipe is flavorful and pairs perfectly with steamed rice. Enjoy your meal!