In a large pot over medium heat, add the olive oil if needed. Once hot, add the diced onion and sauté until it's translucent—about 3 minutes. Then stir in the minced garlic and cook for another minute until fragrant.
Add the Italian sausage to the pot. Break it up with a wooden spoon as it cooks until browned—approximately 5 minutes.
Once the sausage is cooked through, add the diced potatoes to the pot along with four cups of chicken broth. Bring everything to a gentle boil.
Reduce heat to low once boiling. Pour in the heavy cream and add chopped kale or spinach. Stir until everything is well combined and let simmer for about 10 minutes until the potatoes are tender and greens are wilted.
Finally, season with salt, black pepper, and red pepper flakes if desired. Ladle into bowls and top each serving with grated Parmesan cheese before serving.