In a large pot, heat the olive oil over medium-high heat. Add the onion and cook, stirring frequently, until the onions are translucent, about 5 minutes. Stir in the carrots, celery, and thyme. Cook, tossing, for 2 to 3 minutes, or until vegetables are heated through. Pour the broth over the vegetables and stir to combine. Cover the pot and reduce the heat to low, simmering for 1 to 2 hours.