Rinse the napa cabbage and scallions under cool running water, then drain them in a colander.

Cut the firm napa stems crosswise into ½-inch (1.3-cm) pieces and the leaves into 1-inch (2.5-cm) pieces, keeping them separate. Thinly slice the scallions, separating the white and light-green parts from the dark-green tops. Peel and finely grate the ginger, then peel and mince the garlic.

Heat the canola oil in the 6-quart pot over medium heat. Add the scallion whites, ginger and garlic and stir for about 30 seconds, just until fragrant. Add the napa stems and cook, stirring, for about 3 minutes.

Pour in the chicken broth and reduced-sodium soy sauce. Increase the heat and bring the broth to a full boil.

Add the frozen mandu and napa leaves, gently separating the dumplings with the slotted spoon. Let the soup return to a boil, then boil for 6 minutes, stirring gently occasionally to prevent sticking. This timing follows the manufacturer's boiling method for the specified US 24-oz fully cooked pork-and-vegetable mandu. Check the centers of several dumplings with the thermometer: they should reach 165°F (74°C). If needed, continue boiling and check again; floating alone does not establish that they are hot enough. If your package directions differ, follow those directions and check the centers.

Turn off the heat and stir in the rice vinegar and toasted sesame oil. Divide the dumplings among four bowls, then ladle over the napa cabbage and broth. Scatter the reserved scallion greens over the bowls and serve promptly.