Begin by thinly slicing the beef sirloin against the grain for optimal tenderness. Chop the garlic, onion, and red bell pepper into bite-sized pieces. Cook your choice of noodles according to package instructions.

In a large skillet, heat 2 tablespoons of vegetable oil over medium-high heat until shimmering. Add the sliced beef, cooking for 3-4 minutes until browned, then set aside.

In the same skillet, add the minced garlic and chopped onion, sauté for 2-3 minutes until onions are translucent.

Stir in the red bell pepper slices and sauté for 3-4 minutes until tender.

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

Pour the sauce mixture into the skillet, stirring to coat the vegetables evenly, and cook for 2 minutes.

Return the cooked beef to the skillet, mixing well, and heat for 1-2 minutes until the beef is heated through.

Add the cooked noodles to the skillet, gently tossing everything together. Cook for 2-3 minutes.

Remove from heat and serve hot, garnishing with sesame seeds and chopped green onions.