Begin by thinly slicing the beef sirloin into bite-sized pieces, then mince garlic and slice onion and red bell pepper. Cook noodles according to package instructions.

In a large skillet, heat vegetable oil over medium-high heat until shimmering. Test with a splash of water; it should sizzle.

Add sliced beef sirloin to the skillet in a single layer. Cook for about 3-4 minutes without stirring, then flip and cook for an additional 1-2 minutes. Remove beef from skillet.

Lower heat and add minced garlic and onion. Sauté for 2-3 minutes until fragrant and onion is translucent.

Stir in sliced red bell pepper and cook for another 2-3 minutes until softened but crunchy.

In a small bowl, combine soy sauce, brown sugar, gochujang, sesame oil, and rice vinegar. Whisk until smooth.

Pour sauce mixture over sautéed vegetables and allow to bubble for 1-2 minutes.

Return cooked beef to the skillet and toss with vegetables and sauce for about 1 minute.

Add drained cooked noodles, mixing until thoroughly coated. Cook for an additional 2 minutes over medium heat.

Garnish with sesame seeds and sliced green onions before serving. Enjoy hot!