How to Make It
Melt the butter in a large pot over medium-high heat. Add the scallion whites and saffron. Cook, stirring, until softened, 1 to 2 minutes. Add the broth, potatoes, 1 teaspoon salt, and 1/4 teaspoon pepper. Cover and bring to a boil. Reduce heat and simmer until the potatoes are tender, 8 to 10 minutes.
Add the cod, corn, and half-and-half. Cover and bring to a simmer. (The fish will cook through as the soup returns to a simmer.)
Serve topped with the scallion greens.