Preheat your oven to 430°F (220°C). Place the tomatoes, onions, diced garlic, and basil sprigs on a baking sheet, drizzle with olive oil, and season with salt and pepper. Roast for about 45 minutes or until they are soft and slightly brown.

Add the roasted tomatoes and onions, chicken broth, balsamic vinegar, tomato paste, and sugar to the pot. Stir everything together and let it simmer for 10-15 minutes, allowing the flavors to meld.

Use an immersion blender or transfer the soup to a regular blender and blend until it has a slightly chunkier texture. Then stir in the heavy cream. Simmer for an additional 5 minutes.

Scatter the cheese evenly on one bread slice. Place the other slice on top to form sandwiches, then generously butter both sides. Cook in a non-stick pan over medium heat until the cheese is melted and the sandwiches are golden and crisp on both sides.

Slice the grilled cheese sandwich and serve alongside the tomato soup.