Begin by slicing the beef sirloin into thin strips for quick cooking. Mince garlic and slice onion and red bell pepper into thin pieces. Cook noodles according to package instructions, drain and set aside.

In a large skillet, heat vegetable oil over medium-high heat. Add sliced beef in a single layer, cooking for 2-3 minutes until browned on one side. Flip and cook for another 1-2 minutes until fully browned.

Remove beef from skillet and set aside. In the same skillet, add minced garlic and sliced onion, stirring for 1 minute until translucent. Then add sliced red bell pepper, cooking for another 3-4 minutes until tender.

In a bowl, whisk together soy sauce, brown sugar, gochujang, sesame oil, and rice vinegar until sugar dissolves. This is the sauce for your dish.

Return cooked beef to skillet with vegetables. Pour prepared sauce over mixture and stir well to coat. Add cooked noodles and toss thoroughly.

Cook everything together for an additional 2-3 minutes, stirring frequently until heated throughout.

Off heat, sprinkle with sesame seeds and chopped green onions before serving.