1. Heat a large pot of water for the pasta.
2. Trim the fennel stalks and reserve any fronds for an optional garnish. Thinly slice the fennel bulb.
3. Heat the oil in a sauté pan. Add the fennel and cook until edges start to brown.
4. Crush the tomatoes with your hands and set aside. Crumble the sausage.
5. Add the garlic and red pepper flakes to the fennel.
6. Add the sausage and cook until browned.
7. Add the tomatoes, pepper, and nutritional yeast. Simmer for 15 minutes.
8. Boil the pasta in salted water. Reserve 1 cup of pasta water before draining.
9. Add the spinach to the sauce and cook until wilted. Add the pasta and toss until well-coated.
10. Season to taste and garnish with basil or parsley.