Melt the butter in a skillet and fry onion, carrot, and celery cubes for 1minutes; keep stirring. Add tomato paste and bake for another 2 minutes. Put the veggies into the crock pot
Add the tomatoes. Pour the vegetable stock into the slow cooker. Put the remaining ingredients: the basil leaves, thyme, and any leaf on top of the ingredients. Turn the slow cooker on high and wait for 3 hours. Remove the bay leaf. Purée the tomato soup with an immersion blender until you have a smooth tomato soup.