In a stockpot, over medium heat, melt the butter and saute the celery, leek, onion, and garlic until softened, about 5 minutes. Add the flour and cook for another minute.

Add the tomatoes, chicken broth, and basil leaves. Bring to a boil, and gently simmer for 15 – 20 minutes, allowing the flavors to blend. Remove from heat and run the mixture through a food mill into a large bowl, or pan. Sometimes, I also use an immersion blender- which makes it super easy with less mess but yields a somewhat chunkier soup. We like it both ways.

Season with salt and pepper, and adjust to taste. Serve warm in bowls, garnished with crostini slices or large croutons.