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. 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.