Butter one side of each slice of bread. Place two slices of cheese between two slices of bread. Grill until cheese is melted.
Cut grilled cheese into bite-size croutons.
In a large pot or Dutch oven, heat olive oil over medium heat. Add onion and cook until softened, about 5 minutes. Add garlic and cook for 1 minute more.
Add diced tomatoes, vegetable broth, basil, salt, and pepper to the pot. Bring to a boil, then reduce heat and simmer for 20 minutes.
Use an immersion blender to blend the soup until smooth.
Serve hot, garnished with grilled cheese croutons.