In a large bowl, mix chicken with ricotta, Parmesan, egg, salt, and pinch of nutmeg until thoroughly combined. Refrigerate until ready to use.

In a large Dutch oven or heavy pot, combine broth, lemon zest, and thyme and bring to a simmer. Add orzo to the broth. Using a spring-loaded 1/4-cup ice-cream scoop or two large spoons, drop meatball mixture into broth (you should have 18 meatballs). Simmer gently until meatballs float and are cooked through, 10 to 12 minutes. Just before serving, add spinach to broth and let wilt; garnish soup with lemon slices.