In a large stockpot, cook sausage over medium-high heat and crumble as it browns. Remove and set aside.
Lower heat to medium and add butter and onions. Cook until onions are translucent and soft around 3 to 5 minutes, then add garlic and cook for another minute. Season with salt, pepper, and crushed red pepper flakes.
Add kale in batches. Add another batch when the previous one wilts down. Season with nutmeg and cook for 3 more minutes until it is bright green.
Add potatoes, cooked sausage, and chicken broth. Bring to a boil over medium-high heat.
Lower heat to medium low and cook until potatoes are fork tender. Season to taste.