Heat the olive oil in a large skillet over medium-high heat. Add the bulk Italian sausage and cook, breaking it apart with a spoon, until browned (about 5-7 minutes).Add the diced onion, carrots, and celery to the skillet with the sausage. Cook, stirring often, until the vegetables are softened (about 5-7 minutes).Add the minced garlic and red pepper flakes (if using). Cook for 1 more minute until fragrant. Transfer the entire mixture from the skillet to the insert of a 6-quart slow cooker.Add the rinsed and drained cannellini beans, chicken broth, dried oregano, dried thyme, and the sprig of fresh rosemary. Stir to combine.Cover and cook on LOW for 6-8 hours or on HIGH for 3-4 hours.When cooking is complete, remove the rosemary sprig. If you want a creamier soup, use an immersion blender to pulse the soup a few times, or mash 1 cup of the soup and return it to the pot.Stir in the chopped kale. Cover and cook on HIGH for another 20-30 minutes, or just until the kale is wilted and tender.Taste and season with salt and black pepper as needed. Serve hot, garnished with grated Parmesan cheese and crusty bread.