Working ahead: Although the soup must be cooked at the last moment, it takes no more than 10 minutes and little effort. You can prepare all the ingredients hours ahead; cover and refrigerate them.
Cooking the soup with eggs: Have soup dishes, and a tureen if desired, warming in a low oven. Bring the broth to a slow bubble in a 3-quart saucepan. In a medium bowl, beat the eggs with the spices and cheese. Stir the mixture into the bubbling broth with a fork making lacy threads. Taste the soup for seasoning. Drop in the spinach and prosciutto, and immediately pour the soup into the tureen or ladle it into soup dishes, and serve while the spinach is still bright green.
Cooking the soup without eggs: Have soup dishes, and a tureen if desired, warming in a low oven. Bring the broth to a slow bubble in a 3-quart saucepan. Add the spices to the broth, and cook for a few moments. Taste for seasoning. Then stir in the spinach, cheese, and prosciutto, and immediately serve in the soup dishes or tureen, while the spinach is still bright green.
From The Splendid Table: Recipes from Emilia-Romagna, the Heartland of Northern Italian Food by Lynne Rossetto Kasper (Morrow, 1992).
Amy Thielen, author of The New Midwestern Table and host of Heartland Table, says when it comes to freshwater fish, “the lack of a crust opens up a world of flavor possibilities.”