Thinly slice the flank steak against the grain and set aside.
Cook the egg noodles according to package instructions, usually boiling for 5-7, then drain and set aside.
Whisk together soy sauce, brown sugar, rice vinegar, cornstarch, and beef broth in a small bowl.
Heat vegetable oil in a skillet over medium-high heat, cook flank steak until browned for 3-4, then remove.
Sauté minced garlic and ginger for 30 seconds, then add sauce mixture and simmer for 2-3 until thickened.
Add cooked noodles, bell peppers, snap peas, and beef back to skillet, toss until well coated and heated through for 2-3.
Garnish with chopped green onions and sesame seeds before serving.