In a small bowl, add the flour and water. Stir until smooth and thick. Set aside.
In a slow cooker, add the tomatoes, onion, carrot, celery, tomato paste, thyme, vegetable broth, heavy cream, and desired amount of salt and pepper. Add the flour & water mixture. Stir to combine the ingredients.
Cover and cook on low for 6-8 hours or high for 3-4 hours.
Once the soup is ready, use an immersion blender to blend until smooth. Add additional salt and pepper if needed.