Cook the spaghetti or linguine according to package instructions until al dente. Drain and set aside.
In a large skillet, heat the vegetable oil over medium heat. Add the minced garlic and red pepper flakes, sautéing for about 1-2 minutes until fragrant but not browned.
Stir in the soy sauce, brown sugar, rice vinegar, and sesame oil. Cook for another minute until the sugar has dissolved.
Add the cooked noodles to the skillet, tossing to coat them evenly in the sauce. Cook for an additional 2-3 minutes, allowing the noodles to absorb the flavors.
Remove from heat and fold in the shredded lettuce and sliced green onions.
Serve warm, garnished with sesame seeds if desired.