Bring a large pot of water to a boil. Add noodles to boiling water and cook, stirring occasionally, until al dente, 8–10 minutes.

Reserve ½ cup cooking water, then drain pasta, rinse under cold water, and drain well again.

Trim scallions, then thinly slice on an angle, keeping dark greens separate.

Pat chicken dry, then thinly slice crosswise into strips; season with a pinch each of salt and pepper.

In a liquid measuring cup, whisk to combine all of the stir-fry sauce, reserved cooking water, and 1 teaspoon vinegar.

In a large skillet, heat 1½ tablespoons of oil over medium-high heat. It’s ready when the oil is very hot and shimmering. Add the chicken and cook, undisturbed, until it’s browned on one side, about 3 minutes. Stir and continue cooking until the chicken is cooked through, about 2 minutes more. Transfer to a bowl; wipe out skillet.

In the same skillet, heat 1 tablespoon of oil. Add cabbage blend, scallion whites and light greens, chopped garlic, and a pinch each of salt and pepper. Cook, stirring, until cabbage is wilted and slightly softened, about 1 minute. Transfer to bowl with
chicken; return skillet to stove over medium heat.

Add noodles and sauce mixture to same skillet, tossing to coat. Cook until noodles
are warmed through and evenly coated in sauce, about 2 minutes. Add chicken and
cabbage to skillet, tossing to combine. Season to taste with salt and pepper.

Serve chicken lo mein topped with remaining scallion dark greens.