Bring a large pot of salted water to a rolling boil. Cook noodles according to package directions until al dente, then drain, reserving 60ml of pasta water.

While noodles cook, combine the soy sauce, rice vinegar, sesame oil, and brown sugar in a small bowl and whisk until sugar dissolves.

Heat vegetable oil in a small saucepan over medium heat until shimmering, about 2 minutes. Remove from heat and carefully add the minced garlic and chili flakes, letting them sizzle in the hot oil for 30 seconds.

Place drained noodles in a large mixing bowl. Pour the hot chili garlic oil over the noodles immediately.

Add the soy sauce mixture and toss everything together vigorously. Add reserved pasta water one tablespoon at a time if needed for better coating.

Divide noodles among four bowls and top with sliced spring onions, toasted sesame seeds, and julienned cucumber if desired. Serve immediately while hot.