1. Heat the oil in a Dutch oven set over medium heat. Add the onion and crushed red pepper flakes and cook until the onions begin to soften, 5 minutes. Meanwhile, tie the parsley sprigs, lemon zest, and fennel tops together. Add the herb bundle, broth, 2 cups of water, and, if using, the cheese rind. Bring to a very gentle simmer and simmer until the chicken is just cooked through, about 8 minutes. Transfer the chicken to a cutting board and set aside until cool enough to handle. Remove the meat from the bones in large strips, and set aside.
2. Meanwhile, add the sliced fennel, carrots, and celery to the broth and season with salt, to taste. Continue to simmer until the vegetables are just tender, 5 minutes. Stir in the noodles and cook until just al dente, about 5 minutes. Stir in the reserved chicken, baby spinach, and Parmesan until the chicken is heated through and the spinach is wilted. Discard the herb bundle and cheese rind. Stir in lemon juice, to taste. Ladle the soup into serving bowls and serve with additional Parmesan for passing.
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