Heat the vegetable oil in a large pot over medium-high heat. Add the celery, mushrooms and curry powder; cook, stirring, until the curry powder is toasted, 1 to 2 minutes. Add the chicken broth, coconut milk, 1 tablespoon fish sauce and 1 cup water and bring to a simmer.
Add the green beans and bell pepper to the pot, reduce the heat to medium low and simmer until the vegetables are crisp-tender, about 3 minutes. Add the dumplings and cook until tender, about 4 minutes. Stir in the lime juice. Season the soup with salt and add more fish sauce to taste, if desired. Top with sliced scallions and serve with lime wedges.
Photograph by Antonis Achilleos