Place the first 5 ingredients in a 6-qt. stockpot; bring to a boil. Reduce heat; simmer, covered, 10 minutes., Return to a boil. Add tortellini; cook, uncovered, until meatballs are heated through and tortellini are tender, 3-5 minutes, stirring occasionally. Stir in spinach until wilted. Serve immediately. If desired, top with cheese.