Heat oil over medium-high heat in a large stockpot or Dutch oven. Sauté onion, carrots, and celery until beginning to soften, about 7 minutes. Add garlic and cook for 1 minute. Sprinkle flour over the vegetables and cook for 1 minute, stirring continually. Slowly stir in the broth and bring to boil. Reduce heat and simmer, partially covered, for 10 minutes.