Step 1

Stir ginger, garlic, and two-thirds of scallions in a large bowl.

Step 2

Heat grapeseed oil in a small saucepan over high until oil is shimmery and hot but not smoking, about 2 minutes. Pour hot oil over scallion mixture. The scallions will sizzle, turn bright green, and wilt almost immediately. Let sit 5 minutes before stirring in remaining scallions. Stir in soy sauce, vinegar, pepper, sesame oil, and sugar; season with salt. Let sit to let flavors meld, 15 minutes. Taste and adjust seasonings.

Step 3

Meanwhile, cook noodles according to package directions. Drain and toss with as much or as little scallion sauce as you'd like in bowl.

Step 4

Divide noodles among bowls. Top with sesame seeds and serve with chili oil alongside.