Bring a large pot of water to a boil. Add the salt and cook the noodles according to the package directions until just tender. Reserve 1/2 cup (120 ml) of the cooking water before draining. Do not rinse the noodles.

In a small bowl, whisk together the soy sauce, brown sugar, vegetable broth, hoisin sauce, rice vinegar, and sesame oil until the sugar is mostly dissolved.

In another small bowl, stir together the cornstarch and cold water until completely smooth. Set aside.

Heat the neutral oil in a large skillet or wok over medium heat. Add the garlic and ginger and cook for 30 to 45 seconds, stirring constantly, until fragrant. Do not allow the garlic to brown.

Pour the soy sauce mixture into the skillet: Bring it to a gentle simmer for about 2 minutes, stirring occasionally.

Stir the cornstarch slurry again, then whisk it into the simmering sauce. Continue cooking for 1 to 2 minutes until the sauce becomes glossy and lightly coats the back of a spoon.

Add the drained noodles to the skillet: Toss continuously with tongs until every strand is coated. If the sauce becomes too thick, add reserved noodle water 1 tablespoon at a time until the noodles are evenly coated.

Remove the skillet from the heat: Toss in the sliced green onions, reserving a small amount for garnish. Sprinkle with sesame seeds and crushed red pepper flakes, if desired, and serve immediately.