Bring a large pot of salted water to a boil, then cook noodles according to package directions until al dente and drain.

Heat vegetable oil in a large skillet or wok over medium-high heat, add ground beef, and cook until browned, then drain any excess fat.

Add minced garlic and grated ginger to the pan with the beef and sauté for 1-2 minutes until fragrant, stirring constantly.

Whisk together soy sauce, water, brown sugar, rice vinegar, sesame oil, and red pepper flakes in a bowl, then dissolve cornstarch in 1 tablespoon of cold water in a separate small bowl.

Pour the sauce mixture into the skillet, bring to a simmer, then slowly stir in the cornstarch slurry and cook for 1-2 minutes until thickened and glossy.

Add the cooked noodles to the skillet with the sauce and beef, tossing until fully coated.

Divide the noodles into bowls, garnish with sliced green onions and toasted sesame seeds if desired, and serve immediately.