Heat the olive oil in a large pot over medium heat. Add the onion, carrots, celery, parsnip, and yellow bell pepper. Cook for 5–7 minutes, stirring occasionally, until the vegetables are softened.
Stir in the garlic, thyme leaves, and chopped rosemary. Cook for 1–2 minutes until fragrant.
Pour in the vegetable broth, then add the miso paste and bay leaf. Bring to a boil, then reduce heat and let it simmer for 15–20 minutes, until the vegetables are tender.
Remove about half of the vegetables and set them aside. Use an immersion blender to blend the remaining soup until smooth (or carefully transfer to a blender). Return the reserved vegetables to the pot.
Stir in the cooked pastina pasta and allow it to heat through. Add the lemon juice and parsley, then season with salt and pepper to taste.
Notes
Refrigerator: Store any leftover Italian penicillin soup in an airtight container in the refrigerator for up to 3 days.
Freezer: The soup can also be frozen for up to 3 months in a freezer-safe container or zip-top bag. Thaw in the refrigerator before reheating.
To reheat: Transfer the desired portion of soup to a pot and heat over medium-low until warmed through or heat the soup in the microwave. The soup will thicken during storage, so you may want to add water to thin it out a bit.