In a large saucepan, heat the olive oil. Add the sausage and stir to break up the meat. Cook over moderate heat, stirring occasionally, until the sausage is cooked through and lightly browned, about 8 minutes. Transfer the sausage to a plate and crumble.
Add the onion and garlic to the saucepan and cook over moderately low heat, stirring, until softened, about 8 minutes. Add the tomatoes, chicken stock and bay leaf, cover and simmer until the tomatoes have softened, about 20 minutes. Discard the bay leaf.
Transfer the soup to a blender. Add the buttermilk and puree. Return the soup to the saucepan along with the sausage and season with salt and pepper. Simmer over low heat until the sausage has softened, about 10 minutes. Ladle the soup into bowls, garnish with the croutons and serve.