Heat the vegetable oil in a large pot over medium-high heat.
Add the thinly sliced beef and sauté until browned, about 3-4 minutes. Transfer to a plate and set aside.
In the same pot, add the chopped onion, minced garlic, and grated ginger. Cook for 2-3 minutes until the onion is soft and translucent.
Pour in the beef broth and water, bringing the mixture to a vigorous boil.
Add the carrots, bell peppers, sugar snap peas, and shiitake mushrooms. Lower the heat and simmer for about 10 minutes until the vegetables are tender.
Return the cooked beef to the pot, then stir in the soy sauce, sesame oil, and crushed red pepper flakes if desired.
Season with salt and black pepper to taste, and let it simmer for another 5 minutes to meld the flavors.
Serve hot, garnished with chopped green onions and fresh cilantro if desired.