Heat olive oil in a large saucepan over medium heat. Add onions and sauté until soft, about 5 minutes.

Stir in garlic and cook for 1 minute until fragrant.

Add crushed tomatoes, broth, sugar, basil (if using), salt, and pepper. Simmer for 15 minutes.

Use an immersion blender (or transfer carefully to a blender) to puree the soup until smooth.

Stir in cream and simmer for another 5 minutes. Adjust seasoning as needed.

Meanwhile, butter one side of each bread slice. Place cheese between two slices, buttered sides out, and grill in a skillet until golden and cheese melts.

Let the sandwich cool slightly, then cut into small crouton-sized cubes.

Ladle the soup into bowls and top with grilled cheese croutons. Serve hot.