Cook the egg noodles according to package instructions. Drain and set aside.

In a large skillet or wok, heat the vegetable oil over medium-high heat. Add the diced onion and cook for 2-3 minutes until softened.

Add the minced garlic and cook for an additional 30 seconds until fragrant.

Add the ground beef to the skillet, breaking it apart with a spatula. Cook until browned and cooked through, about 5-7 minutes.

Stir in the sliced bell pepper and broccoli florets. Cook for another 3-4 minutes until the vegetables are tender.

In a small bowl, whisk together the soy sauce, brown sugar, rice vinegar, ground ginger, and red pepper flakes (if using). Pour this sauce over the beef and vegetables in the skillet.

Add the cooked egg noodles to the skillet and toss everything together until well combined and heated through, about 2-3 minutes.

Serve hot, garnished with sliced green onions.