"Save your cauliflower greens! They are edible! You may not expect much from a bowl of greens, but this soup is savory, delicious, and paleolithic. Garnish each serving with Parmesan cheese and parsley if desired."
Combine cauliflower stems and leaves, chicken broth, garlic, and shallots in a 4-quart pot; add enough water to cover the leaves. Cover pot and bring liquid to a simmer.
Heat bacon drippings in a skillet over medium heat. Cook and stir onion in the hot bacon drippings until translucent, 5 to 10 minutes. Add onion to the soup and continue to simmer until stems soften, about 2 hours.
Pour soup into a blender no more than half full. Cover and hold lid down; pulse a few times before leaving on to blend. Puree in batches until smooth. Return soup to pot and stir spinach, salt, and pepper into soup; simmer for 5 minutes.