In a large stockpot over medium heat, melt the butter and oil. Add the onion and celery and saute for 6-8 minutes, until softened. Add the garlic and thyme and saute, stirring frequently, for 1 minute, until fragrant.

Stir in the carrots. Add the water and broth and bring to a boil. Reduce to a smaller and cook for 40 minutes, until the carrots are soft enough to puree.

Use an immersion blender to puree the soup until smooth and silky. Season with salt and pepper. Stir in the half-and-half. Reheat if necessary and serve immediately.