1. Begin by preparing the rice noodles according to the package instructions. Once cooked, drain and set aside.
2. In a small bowl, mix the soy sauce, brown sugar, mirin, and the cornstarch slurry. This will be your teriyaki sauce.
3. Heat sesame oil in a large skillet over medium-high heat. Add the sliced flank steak and cook until browned, about 2-3 minutes on each side.
4. Add the minced garlic and grated ginger to the pan, stirring for about 1 minute until fragrant.
5. Pour the teriyaki sauce into the skillet, stirring to coat the beef. Let it simmer for 2-3 minutes until it thickens.
6. Add the broccoli florets and red bell pepper to the skillet, cooking until they are tender-crisp, about 4-5 minutes.
7. Toss in the cooked rice noodles, ensuring they are evenly coated with the sauce.
8. Garnish with chopped green onions and sesame seeds before serving.

When cooking the beef, aim for a quick sear to keep it tender. My tip is to slice it as thinly as possible, almost akin to the way my grandmother used to slice meat for fajitas, ensuring every piece absorbs the rich sauce.