Preheat oven to 300°.
Combine sourdough bread and 1 tablespoon olive oil in a medium bowl; toss gently to coat. Spread bread cubes in a single layer on a jelly roll pan; bake at 300° for 20 minutes, stirring once.
Heat 1 teaspoon olive oil in a medium saucepan over medium-high heat. Add onion and garlic; sauté 3 minutes or until browned. Add broth, basil, and tomatoes. Bring to a boil; cover, reduce heat to medium, and simmer 10 minutes.
Ladle soup into each of 5 soup bowls. Top evenly with croutons and Parmesan cheese.