1. Heat a skillet over medium heat. Butter one side of each bread slice. Place 4 slices, butter-side down, on the skillet. Top each with a slice of cheese and another slice of bread, butter-side up. Cook until golden brown, then flip and cook until the other side is golden and the cheese is melted.
2. In a saucepan, heat olive oil over medium heat. Add chopped onion and cook until softened. Add minced garlic and cook for another minute.
3. Add canned tomatoes and chicken broth to the saucepan. Bring to a boil, then reduce heat and simmer for 15 minutes.
4. Use a blender to puree the soup until smooth. Return to the saucepan, season with salt and pepper, and heat through.
5. Serve the grilled cheese sandwiches with the tomato soup.