In a large saucepan, heat 1/4 cup extra virgin olive oil over medium heat. Add the onions, carrots and celery, along with 1 teaspoon salt and ½teaspoon pepper. Cook, stirring occasionally, until the vegetables are tender, 8-10 minutes. Add the garlic and cook, stirring, 30 seconds. Add the sweet potatoes, bay leaves, thyme sprigs and remaining ½ teaspoon salt, along with 5 cups water. Bring to a boil. Reduce heat to simmer and cook, partially covered, 10-15 minutes, or until sweet potatoes and vegetables are tender.