In a large pan, heat oil over a medium high heat. Add onions, carrots and celery, garlic and mushrooms, and saute 2-3 minutes until onions are translucent.
Add pork, oregano, fennel, red pepper flakes, salt and pepper, and continue stirring until meat begins to brown, about 5 minutes.
Add tomatoes and broth and continue stirring until soup comes to a boil. Reduce heat and simmer until carrots are nearly tender, about 15 minutes. Add sliced zucchini and cook another 5 minutes or until zucchini is tender. (Add more broth if desired)
Turn off heat and adjust seasoning with salt, pepper and more red pepper flakes if desired. Stir in baby spinach to wilt.
Serve in bowls with pistou. Stir 1-2 Tablespoons pistou into each bowl just before serving.