In a large pot or Dutch oven, heat 3 tablespoons (45 ml / 1.5 fl oz) of the olive oil over medium heat.
Add the chopped onion, carrot, and celery.
Sauté for about 5 minutes, stirring occasionally, until the onion has softened.
Add the diced hard vegetables to the pot, sprinkle generously with salt and pepper, and stir for another minute.
Pour in the vegetable stock and the chopped tomatoes.
Bring to a boil, then reduce the heat so the soup maintains a gentle simmer.
Cook for about 15 minutes, until the hard vegetables are beginning to soften. (The soup can be made ahead to this point and refrigerated for up to 2 days.)
Stir in the soft vegetables and the fresh parsley.
Continue to simmer for another 15 minutes, or until all the vegetables are perfectly tender.
Taste and adjust the seasoning with more salt and pepper if needed.
Stir in the remaining 1 tablespoon (15 ml / 0.5 fl oz) of olive oil and serve hot, passing Parmesan cheese at the table if desired.
Don't throw away your Parmesan rind!
Add a piece to the soup along with the hard vegetables. It will infuse the broth with a rich, savory flavor and become delightfully chewy.
