In a large pot or Dutch oven, heat the olive oil over medium heat. Add the Italian sausage and cook until browned, breaking it up with a spoon, about 5-7 minutes.
Add the diced onion, garlic, carrots, and celery to the pot. Sauté until the vegetables are softened, about 5 minutes.
Stir in the oregano, thyme, and red pepper flakes (if using) and cook for another minute until fragrant.
Pour in the chicken broth and bring the mixture to a boil.
Add the white beans and reduce the heat to a simmer. Let it cook for 15 minutes to allow the flavors to meld.
Stir in the chopped Tuscan kale and cook for an additional 5 minutes, or until the kale is wilted and tender.
Season the soup with salt, pepper, and lemon juice to taste.
Serve hot, garnished with grated Parmesan cheese if desired.