Prep all ingredients according to specifications above.
Heat oil in a 6-quart (or larger) pot or Dutch oven over medium-high heat. Add onions, celery, carrots, fennel, garlic and crushed red pepper and cook for about six minutes, or until vegetables have softened a bit, stirring occasionally.
Add 2 cups broth and stir to deglaze, loosening and scraping up any browned bits on bottom of pot. Then, add remaining broth, tomatoes, bean cooking liquid, potatoes (drain first if being held in water), rosemary sprigs, cheese rind, salt and black pepper and stir until all ingredients are well-combined.
Cover pot, increase heat and bring mixture to a boil. Then, immediately reduce heat to a simmer and simmer, partially covered, for about 10 minutes, stirring regularly. Stir in kale and continue to cook until the potatoes are soft, about 15 to 20 more minutes. The soup should be rather thick; add more broth if needed. Stir in beans about 5 minutes before the soup is done. Remove the rosemary stems and what’s left of the parm rind, then taste and adjust seasonings and turn off heat.
Arrange a layer of bread in a large glass or stainless-steel bowl, then ladle some soup over the bread. Repeat this layering of bread and soup until all the bread and soup are used up, finishing with a layer of soup. Press the bread down to ensure that it is all immersed in the soup. Let this cool for about 30 minutes, then cover and refrigerate overnight (or a minimum of 4 hours).
The following day, serve the ribollita. In can be reheated in one of two ways. The first and my favorite way is to heat some olive oil in a skillet, then add some now very thick, stew-like soup to it and heat until an irresistible, thin crust forms. The second way is to heat the ribollita in a pot in a more traditional way, with a bit more liquid (broth or water) added. With either reheating method, some high-quality extra-virgin olive oil should be drizzled over each serving with a sprinkling of either parmigiano or pecorino cheese for an additional hit of flavor and richness.
ALTERNATIVE (QUICKER) METHOD: Slice the bread slices into cubes, then fold bread cubes into the soup about 5 minutes after the addition of the beans. Continue to simmer the soup for about 5 to 10 more minutes while stirring. The bread will break apart and further thicken the soup. Then, ladle the soup into bowls, topping each portion with a generous drizzle of high-quality, extra virgin olive oil and a sprinkling of either parmigiano or pecorino cheese. Buon Appetito!