Preheat oven to 300ºF. In a large bowl, mix 1/2 cup olive oil and 3 minced garlic cloves. Add bread pieces and toss to coat. On a large baking sheet, arrange coated bread in a single layer. Bake in preheated oven until golden (approximately 20 minutes), stirring once. Set baked croutons aside.
TOMATO SOUP (can be started while croutons are baking):
In a large heavy-duty pot, heat 2 tbs. olive oil over medium heat. Add onions and sauté for approximately 25 minutes, or until very tender. Add remaining six minced garlic cloves and cook for one minute. Add chicken stock, crushed tomatoes and 1/2 cup grated Parmesan. Simmer uncovered for about 30 minutes, stirring occasionally. Season with salt and fresh ground pepper.
Add garlic bread croutons to soup and cook for one minute more. Ladle soup into bowls and garnish each serving with basil slices and Parmesan shavings.