Heat butter in a large soup pot over medium heat. Add the onion, and cook until soft. Then add the sausage and cook until browned.
Add in the garlic, fennel, red pepper flakes, Italian seasoning and salt and stir well. Add in diced tomatoes and chicken broth. Bring to a boil, then reduce heat and simmer for 10 minutes.
Add in the gnocchi and kale and continue cooking for another 5-7 minutes. Remove from heat and serve with parmesan cheese.
Notes
Store in an airtight container in the fridge for up to 4 days, or in the freezer for up to 3 months. Reheat on the stove over medium low heat.