Preheat oven to 300° F.
Mix 1/2 cup olive oil and 3 minced garlic cloves in large bowl. Add sourdough bread pieces and toss to coat. Arrange bread in single layer on large baking sheet. Bake until golden, stirring once, about 20 minutes. Set garlic-bread croutons aside.
Heat remaining 2 tablespoons olive oil in heavy large pot over medium heat. Add onions and sauté until very tender, about 25 minutes. Add remaining minced garlic and cook 1 minute. Add chicken stock, crushed tomatoes and 1/2 cup Parmesan cheese. Simmer soup uncovered 30 minutes, stirring occasionally. Season to taste with salt and pepper.
Add garlic-bread croutons to soup and cook 1 minute. Ladle soup into bowls. Sprinkle with thinly sliced basil and Parmesan cheese shavings.Makes 6 Source: