As the meatballs are cooking, begin preparing the soup. Heat the butter and olive oil in a large pot or Dutch oven over medium-high heat. Add the onion and cook for approximately 3-4 minutes, or until soft and translucent. Mix in the carrots and celery and continue to cook for 6-8 minutes, stirring often. Add the garlic and salt and sauté for an additional 1 minute, stirring continuously to prevent the garlic from burning.