In a large pot over medium heat, saute the onion, celery, carrot, and garlic in the oil until tender, about 10 minutes.
Add the remaining ingredients and bring to a boil. Reduce heat and simmer until vegetables are tender, about 20 minutes.
Use an immersion blender to puree the mixture until smooth, or ladle a small amount into a blender and blend until smooth. If you are using a blender or food processor, only work with small amounts of soup at a time. Too much hot liquid in your blender may cause the top to blow off while blending. That's a horrible mess to clean up.
Serve with gluten-free croutons, or gluten-free grilled cheese sticks. Enjoy!