Heat 3 Tbsp. extra-virgin olive oil in a medium Dutch oven or other heavy pot over medium-high. Add 2 large onions, finely chopped, and 1 small fennel bulb, thinly sliced (reserve fronds), and cook, stirring often, until softened and deeply browned and frizzled in spots, 10–14 minutes. (You should see some browning on the pot at this point.) Add 8 garlic cloves, finely grated, and ¼ tsp. crushed red pepper flakes and cook, stirring constantly, just until fragrant, about 1 minute. Add 1 Parmesan rind (if using), 4 cups low-sodium vegetable broth, and 5 cups water and bring to a boil. Reduce heat to low so soup is at a very gentle simmer.

Mix together 1 large egg yolk, 1½ oz. Parmesan, finely grated (about ¾ cup), 1 cup whole-milk ricotta, ½ cup plain dry breadcrumbs (not panko), ¼ cup finely chopped parsley, 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, and ¼ tsp. freshly ground black pepper in a large bowl. Using a #60 cookie scoop (about 1 Tbsp.), drop balls of ricotta mixture into soup and cook until they float to the surface, 5–7 minutes.

Add ⅔ cup ditalini or other small tubular pasta and simmer until still very al dente, about 4 minutes. Add 1 small head of escarole, cut into 1" pieces, and stir to wilt, then stir in ½ tsp. finely grated lemon zest and 2 Tbsp. fresh lemon juice. Taste soup and season with more salt and black pepper if needed.

Ladle soup into bowls and top with parsley leaves, reserved fennel fronds, and more finely grated Parmesan.