Thinly slice the beef sirloin against the grain and prepare vegetables.

Heat vegetable oil in a large skillet over medium-high heat.

Sauté sliced beef until browned and cooked through, then set aside.

Add minced garlic and sliced onion to the skillet, sautéing until fragrant.

Add sliced red bell pepper and cook until slightly softened.

In a bowl, combine soy sauce, brown sugar, gochujang, sesame oil, and rice vinegar to create the sauce.

Return beef to the skillet, pour the sauce over, and stir to combine.

Add cooked noodles to the skillet, gently tossing to blend with the sauce.

Serve garnished with sesame seeds and sliced green onions.