For the sauce and chicken:
1/4 cup tamari or soy sauce
2 tablespoons Shaoxing wine or dry sherry
1 1/2 teaspoons cornstarch
1 teaspoon granulated sugar
1/4 teaspoon kosher salt, plus more for cooking the noodles
1 medium boneless, skinless chicken breast (about 8 ounces)
For the noodles:
8 ounces dried lo mein or udon noodles
3 tablespoons vegetable oil, divided
1/2 medium yellow onion, thinly sliced
Kosher salt
4 ounces snow peas or sugar snap peas, halved on a slight diagonal
1 medium red bell pepper, thinly sliced
1 clove garlic, minced
1 scallion, thinly sliced (optional)