Slice the beef thinly against the grain. In a bowl, combine beef slices with 2 tablespoons soy sauce and cornstarch. Mix well and let marinate for about 15 minutes.

Prepare the Sauce

In a small bowl, mix together the remaining soy sauce, minced garlic, minced ginger, and water. Set aside.

Stir-Fry the Beef

In a large skillet or wok, heat 1 tablespoon of vegetable oil over medium-high heat. Add the marinated beef and stir-fry for about 2-3 minutes until browned. Remove the beef from the pan and set aside.

Cook the Vegetables

In the same pan, add the remaining tablespoon of vegetable oil. Add broccoli florets and shiitake mushrooms, stir-frying for about 4-5 minutes until tender-crisp.

Combine and Serve

Return the beef to the pan with the vegetables. Pour the sauce mixture over the beef and vegetables, tossing to combine. Cook for another 2-3 minutes until everything is heated through. Season with salt and black pepper to taste. Serve hot over rice or noodles.