Heat 2 tablespoons oil in Dutch oven over medium-high heat until shimmering. Add onion, garlic, red pepper flakes (if using), and bay leaf. Cook, stirring frequently, until onion is translucent, 3 to 5 minutes. Stir in tomatoes and their juice. Using potato masher, mash until no pieces bigger than 2 inches remain. Stir in sugar and bread; bring soup to boil. Reduce heat to medium and cook, stirring occasionally, until bread is completely saturated and starts to break down, about 5 minutes. Remove and discard bay leaf.
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