Preheat the oven to 350 degrees F (175 degrees C).

Heat the oil in a large pot over medium heat. Add the onion, garlic, and rosemary and cook for 5 minutes, or until the onion is soft but not brown. Add the escarole and stock. Bring to a boil. Reduce the heat, partially cover the pot, and simmer for about 10 minutes, or until the escarole is tender.

In a medium bowl, mix the turkey, egg, green onion, parsley, and Parmesan cheese. Form into 1-inch (2.5-cm) balls. Lightly oil a baking dish large enough to hold the meatballs in a single layer. Add the meatballs. Bake for 10 minutes.

Add the meatballs to the soup and simmer, partially covered, for 5 minutes. Season to taste with salt and pepper. Serve garnished with lemon slices (if using).