Make the soup
Finely chop the onion and the fresh rosemary leaves (removed from the stem). Wash the escarole,
spin-dry, then cut it crosswise into 1 cm strips. Set aside.
Heat half of the oil in a large saucepan over moderate heat. Add the onion and rosemary, then cook, with stirring, until the
onion is softened, about 5 min. Add the escarole, stir to coat it with oil, then cover and cook 2 min. Pour in the broth, then bring to a boil. Stir in the pasta, then reduce the heat and simmer, covered, until the pasta is cooked al dente,
about 10 min.
Make the meatballs
While the soup simmers, mince the scallions, then put them in a bowl. Add the egg, ground meat, bread crumbs, and freshly grated Parmesan. Season with salt and pepper, then combine well. Form the mixture into meatballs about 2 cm in diameter (about 4 meatballs per serving).
Heat the remaining oil in a skillet over medium-high heat until hot but not smoking. Add the meatballs, then sauté until they are golden-brown on all sides, about 5-6 min.
Finish the soup
Transfer the meatballs to the soup, cover, then simmer
Mince, then mash the garlic, stir it into the soup, then pour in the lemon juice. Adjust the seasoning, then serve the soup into pasta bowls.