Toast the walnut pieces in a 12-inch nonstick skillet over medium-high heat, stirring occasionally, until fragrant, about 3 minutes. Transfer to a plate.
Heat the vegetable oil in the skillet over medium-high heat. Add the onion and season with salt and pepper. Cover and cook, stirring occasionally, until beginning to soften, about 5 minutes.
Meanwhile, in a small bowl, combine the soy sauce, hoisin sauce, sesame oil, and 1 Tbs. water.
Add the scallops and ginger to the skillet and cook, stirring, until the scallops are just opaque, about 1 minute. Add the soy sauce mixture and stir to coat.
Stir in the walnuts, sprinkle with the cilantro, and serve.