Step 1: Prepare Your Ingredients - Thinly slice the beef sirloin, mince garlic, and slice onions and bell peppers. Cook noodles and set aside.

Step 2: Heat the Oil - In a skillet, heat vegetable oil over medium-high until shimmering.

Step 3: Cook the Beef - Add beef to the skillet and brown on one side, about 3-5 minutes. Remove from skillet once cooked.

Step 4: Sauté Aromatics - In the same skillet, add minced garlic and onion, sauté until translucent.

Step 5: Add the Bell Pepper - Cook sliced bell pepper with garlic and onion for 2-3 minutes.

Step 6: Mix the Sauce - Whisk together soy sauce, brown sugar, gochujang, sesame oil, and rice vinegar in a bowl.

Step 7: Combine Sauce with Vegetables - Pour the sauce into the skillet and cook for 2 minutes.

Step 8: Return Beef to Skillet - Add cooked beef back into the skillet and mix.

Step 9: Add the Noodles - Toss cooked noodles with beef and vegetables in the skillet.

Step 10: Finish Cooking - Cook until heated through and glossy, about 2 more minutes.

Step 11: Garnish and Serve - Serve hot, garnished with sesame seeds and green onions.