Preheat the oven to 500 degrees F.
Prepare the tomatoes. In a large bowl, mix the tomatoes, 1/4 cup of the oil, the vinegar, garlic, salt, and pepper, to taste. Spread the tomatoes out on a non-reactive baking sheet. Roast the tomatoes in the oven until very dark in spots, 35 to 40 minutes. Remove and allow to cool a bit.
Prepare the bruschetta. Cut the bread crosswise into slices about 1-inch thick (you will need 8 slices). Lightly brush the slices on both sides with oil and season with salt. Place the slices on a baking sheet and toast in the oven until the bruschetta are golden brown and just beginning to crisp, about 6 minutes.
In a large saucepan over medium heat, combine remaining 1/4 cup oil, the onions, and a pinch of salt. Cook until the onions are very soft, 8 to 10 minutes, stirring occasionally. Add the 2 cups basil leaves and saute with the onions for about 1 minute.
Add the roasted tomatoes and water to the saucepan. Bring the mixture to a simmer and cook for 10 minutes. Season with salt and pepper, to taste. Puree the tomato mixture in a blender. Start the motor at a slow speed and increase gradually. Alternatively you can use an immersion blender right in the pot. The mixture should be very smooth. You should have about 8 cups. You can prepare the soup to this point and refrigerate it. When ready to serve, pour the soup into a medium saucepan and bring it to a slow simmer over medium heat.
Serve the soup. Place 1 bruschetta in the center of each shallow soup bowl. Pour the soup around each bruschetta. Garnish with torn basil leaves.