How to Make It
In a large saucepan, melt the butter. Add the onion and cook over moderate heat, stirring, until softened, about 5 minutes. Add the tomatoes and their juices, along with the tomato paste and cook, stirring, for 5 minutes. Add the broth and basil and season with salt and pepper. Simmer until the tomatoes are broken down, about 15 minutes. Discard the basil and puree the soup until smooth.
Preheat the broiler and arrange the baguette toasts on a baking sheet. Sprinkle with the cheese and broil just until melted, about 30 seconds. Serve the soup with the cheese toasts.