Melt butter over medium heat in a large soup pot or Dutch oven.
Add chopped onions and saute over medium heat for 3 minutes.
Add carrots, celery and garlic and saute for another 5 minutes, stirring occassionaly to prevent burning. A little browning is O.K.
Add herbs and saute for 1 minute more.
Add chicken stock, stirring to deglaze the bottom of the pot if necessary.
Add parmesan rind, if using.
Bring to boil, reduce heat, cover and let simmer for 30 minutes, until carrots are soft.
If you used a parmesan rind, remove it now.
With a slotted spoon, transfer all the veggies to a blender. Add a cup or two of the stock and blend until smooth. When blending hot liquids, let it cool just a bit, start your blender slowly, and use the minimum amount of liquid to avoid hot liquid venting from the blender.
Return the soup to the pot and taste. Add salt and pepper as desired. If your broth seems flat, add 1-2 teaspoons of good chicken bouillon for added flavor.
Add chicken pieces to warm up in the soup broth.
Cook pastina according to package directions.
Add lemon juice to taste right before serving to keep the flavor fresh.
Pour broth to bowls and add cooked pastina. Garnish with freshly grated parmesan cheese, chopped fresh parsley, and/or a dollop of pesto.