Heat 1 tablespoon olive oil in a large pot or Dutch oven over medium-high heat. Add the onion, 3 garlic cloves and a pinch of salt. Cook, stirring occasionally, until the onion is softened, 5 minutes. Add the tomato paste and cook, stirring occasionally, 1 minute. Add all but 1/2 cup of the chopped tomatoes, the beans, broth, 1 cup water, the rosemary sprig, 1/4 teaspoon red pepper flakes and 1/2 teaspoon salt. Bring to a boil. Reduce the heat to medium and simmer 20 minutes.
Discard the rosemary sprig. Working in batches, transfer the soup to a blender and puree until smooth. Season with salt. Return to the pot and cover to keep warm.
Preheat the oven to 400 degrees F. Heat the remaining 2 tablespoons olive oil and 2 garlic cloves in a small skillet over medium heat. Cook, stirring occasionally, until lightly browned, 5 minutes. Toss with the bread, chopped rosemary, mozzarella and a pinch each of salt and red pepper flakes on a baking sheet. Bake, stirring occasionally, until the croutons are golden, 10 minutes. Top each bowl of soup with the croutons and the reserved 1/2 cup chopped tomatoes; drizzle with olive oil.
Photograph by Charles Masters
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.