In a large pot, bring stock up to a boil. Drop in the frozen tortellini. Give it a stir and cook according to package directions.
When tortellini are done and look like puffed up pillows, ladle soup into soup bowls. Wilt a few leaves of spinach into each bowl.
Top with some lemon zest and a small handful of Parmigiano. Serve with some crusty bread alongside.