Bring a large pot of water to a boil. Add thin noodles and cook until al dente per package instructions (about 4–5 minutes). Drain and set aside.

Heat sesame oil in a large skillet over medium heat. Add minced garlic and cook until fragrant and golden (about 1 minute), stirring to prevent burning.

Sprinkle red chili flakes into the skillet. Stir constantly for 30 seconds until aromatic, releasing a spicy, smoky perfume without letting the flakes turn bitter.

Pour in soy sauce, rice vinegar, and brown sugar into the skillet, stirring until sugar dissolves and sauce simmers gently (about 1–2 minutes).

Add the cooked noodles and half of the sliced green onions, then toss vigorously in the pan for 1 minute until every strand glistens with spicy, vibrant sauce.

Transfer the noodles to warmed bowls. Sprinkle with toasted sesame seeds and remaining scallions, top with fresh cilantro if you like, and serve immediately so they stay piping hot.