Cook the bacon in a Dutch oven or large pot over medium-high heat until browned around the edges, about 5 minutes. Add the ginger, half of the scallions, 1 teaspoon salt and the sugar; stir-fry 30 seconds. Add the broth and 1 cup water; cover and bring to a boil. Meanwhile, combine the cornstarch and 3 tablespoons water in a small bowl. Add the sesame oil and 1 teaspoon pepper and stir to combine.
Add the snow peas, tomatoes, watercress and dumplings to the boiling broth mixture. Stir the cornstarch mixture again and add to the soup. Cook, stirring occasionally, until the dumplings are tender and the broth thickens slightly, 3 to 5 minutes. Stir in the remaining scallions and season with pepper.