Combine onion, celery, tomatoes and tomato paste in food processor or blender and blend until smooth.
Add vegetable mixture, broth, salt and pepper to large pot. Bring to a boil over high heat. Reduce to a simmer and cook, stirring occasionally, 10 minutes.
Top each bowl with a sprinkle of basil.
Preheat oven to 350ºF.
Butter one side of each slice of bread. Place two slices, butter-side-down, on parchment-lined baking sheet. Top with cheese and other slice of bread, butter side up.
Bake 5-10 minutes, or until browned and cheese is melted.