If you don't already stash Parmesan rinds in your freezer, look for them at a supermarket with a specialty cheese section—ask for them if they're not prepackaged. You'll need six to eight rinds.
To prepare soup: Bring water to a boil in a large pot over medium-high heat. Wrap fennel stalks, bay leaves, thyme, peppercorns and orange zest in cheesecloth and tie with kitchen string. Add to the pot along with Parmesan rinds. Adjust heat to maintain a simmer, partially cover and cook for 30 minutes.
Stir in diced fennel, tomato, carrot, green beans, leek, white beans and crushed red pepper. Partially cover and simmer until the vegetables are very tender, about 30 minutes.
Meanwhile, prepare pistou: Combine basil, mint, grated Parmesan, garlic and crushed red pepper in a food processor. Process until the herbs are finely chopped. Scrape down the sides. With the motor running, stream in oil and process until mostly smooth.
Remove the Parmesan rinds and sachet from the soup. Stir in escarole and spinach; cook until wilted, about 5 minutes. Season with salt and pepper. Serve topped with the pistou.
Cheesecloth, kitchen string