Soak the 15-bean mix overnight. Drain and rinse thoroughly before cooking. Set aside.
In a large Dutch oven or stockpot over medium-high heat, cook the Italian sausage, breaking it apart as it browns. Once fully cooked, remove with a slotted spoon and set aside.
In the same pot, add the chopped onions, red bell peppers, and garlic. Cook for 5–7 minutes, stirring occasionally, until the onions are translucent and the peppers are softened.
Stir in the fennel seeds, smoked paprika, oregano, and crushed red pepper flakes. Cook for 1–2 minutes to toast the spices and deepen the flavor.
Add the soaked beans, crushed tomatoes, and chicken broth. Stir well and bring to a gentle simmer. Cook uncovered for 1½ to 2 hours, or until the beans are tender, stirring occasionally.
Return the cooked sausage to the pot. Stir in the sherry or red wine vinegar and simmer for an additional 15–20 minutes. Adjust seasoning with salt and black pepper to taste.
Ladle into bowls and top with Parmesan, shredded cheese, or fresh basil if desired. Serve hot and enjoy.