
Melt the butter in a medium pot over medium heat. Add the onion, carrot, celery, salt, and pepper. Cook for 5–6 minutes, stirring often, until the vegetables are soft and fragrant.
Pour in the broth and bring to a gentle boil.
Add the pastina and cook according to package directions, about 12–15 minutes, or until tender.
Turn off the heat and stir in the Parmesan cheese, lemon zest, and lemon juice. Serve warm, topped with extra Parmesan and chopped fresh parsley.