In a large soup pot over medium-high heat, cook the Italian sausage pieces until they're nicely browned, about 5 minutes. Remove and set aside.
In the same pot, add the chopped onion, minced garlic, cubed potatoes, Italian seasoning, and dried chili flakes. Sauté until the onion is translucent, around 5 minutes.
Pour in the beef bone broth and return the browned sausage to the pot. Bring to a boil, then reduce heat to a simmer. Let it cook until the potatoes are fork-tender, about 8-10 minutes.
Stir in the chopped kale or swiss chard until wilted. Season the soup and serve warm.