Heat olive oil and butter in a large stock pot over medium heat. Add carrots, onions, celery, garlic and salt and cook for 5 minutes, or until onions are translucent. Do not let vegetables brown. Add one quart of vegetable stock and bring to a boil. Reduce heat, cover and simmer until vegetables are tender, about 20 minutes. Remove from heat and allow to cool for 10 minutes.

Using an immersion blender, a blender, or a food processor, puree soup in batches until completely smooth.  Take care not to over fill the  blender and make sure to vent the lid so the soup doesn't explode. Return soup to pot and add orange zest and orange juice and stir  well. Taste for seasoning and add more salt and pepper if necessary. If soup is too thick, add additional broth until it is the right consistency. Drizzle with cream or a dollop of plain yogurt if desired.