In a Dutch oven, combine the prosciutto, Parmesan rind, thyme, carrots, celery, garlic, shallots and 7 cups water. Bring to a simmer over medium-low heat. Simmer for 45 minutes, skimming any foam comes to the surface. Strain the broth and set aside.
Meanwhile, in a small saucepan, add the farro and water to cover by 1 inch. Add the salt and bring to a simmer over medium heat. Cover and simmer until the farro is tender and most of the water is absorbed, about 20 minutes. Drain off any excess water and set aside.
In a separate Dutch oven or large saucepan, heat the olive oil over medium-high heat. Add the onions and reduce the heat to medium. Cook, stirring often, until deep golden brown, 10 minutes. Deglaze with the Marsala. Add the cooked farro and the prosciutto broth, and bring to a simmer. Add the spinach and stir until wilted, 2 to 3 minutes.
Serve the soup in bowls. Top with Parmesan crisps if using.
To make the Parmesan crisps, preheat the oven to 375 degrees F and line a baking sheet with parchment paper. Measure 1/4 cup grated Parmesan in small mounds on the parchment and spread 1/8-inch thick. Bake until bubbly and beginning to brown, 9 minutes. Allow to cool slightly on the baking sheet before removing to a wire rack to cool completely.