In a large pot, heat the olive oil over medium heat. Add the sausage and cook, breaking it up into crumbles, until browned, about 5-6 minutes. Drain any excess fat if needed.
Add the diced onion, garlic, carrots, and celery to the pot. Sauté for 4-5 minutes until the vegetables are softened.
Stir in the Italian seasoning and red pepper flakes (if using), cooking for 1 minute until fragrant.
Pour in the chicken or vegetable broth and add the white beans. Stir to combine and bring the soup to a boil.
Reduce the heat to a simmer, cover, and cook for 15-20 minutes to allow the flavors to meld.
Add the chopped kale and cook for an additional 5 minutes, or until the kale is wilted.
Season with salt and pepper to taste. Serve hot, garnished with fresh parsley if desired.