In a 2-qt saucepan, heat the oil over medium-high heat. Add the onion and cook, stirring, until slightly softened, about 2 minutes. Add the broth and water and bring to a boil;.
Add the zucchini, dill, and poultry seasoning and return to a boil. Reduce heat and simmer, uncovered, 5 minutes. Add the escarole through the salt. Simmer, uncovered, 5 minutes longer. Sprinkle with the scallion to serve.