Melt butter in a large pot. Add sausage and onions and cook until brown and onions softened (about 6-8 minutes). Add garlic and cook for 1 more minute.
Pour in chicken broth, add chili flakes and bring to a boil. Cover and simmer for 5 minutes.
Stir in spinach and cream. Season with salt and pepper.
Notes
Do not let the cream come to a boil so that it doesn't curdle. Simply add the cream at the end of the cook time, when the temp is very low, and allow it to heat through while the spinach wilts.