In a big soup pot, heat the oil over medium-high heat. Add the onion and cook, stirring, until softened, 6 to 8 minutes. Add the carrots, celery, garlic, thyme, 1 tbsp. salt and 1 tsp pepper and cook, stirring, until the carrots soften slightly, about 5 minutes. Add the bay leaf and chicken broth and bring to a boil. Reduce the heat and simmer for 45 minutes to meld the flavors.