Add onion, carrots, celery, garlic and chicken stock in a stock pot.
Bring to a boil over medium-high heat. Reduce the heat and simmer until vegetables are tender, about 10-15 minutes.
Remove the veggies with a slotted spoon and place in a blender. Add 1 cup of cooking liquid and blend until smooth.
Assemble the soup:
Pour the blended veggies back into the stock pot and add pastina pasta.
Add thyme and season to taste with salt and pepper.
Simmer the soup until the pasta is cooked, stirring frequently. It usually takes about 18-20 minutes. Add the chicken in the last 5 minutes of cooking.
Remove from heat and discard the thyme sprigs.
Stir in the parmesan.
Serve:
Serve the soup warm, garnished with chopped parsley.
Notes
You can add a little more protein to this soup by adding some whisked eggs during the last minute of cooking.