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Turn oven on to broil. Line a baking pan with parchment paper.
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Heat the balsamic in a pan over medium-high heat. Bring to a boil and reduce heat to simmer. Stir occasionally until the balsamic has reduced and thickened.
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Cut the grapefruit in half. Sprinkle both sides with cinnamon and honey, if desired. Broil for 5 minutes.
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Top the grapefruit with the balsamic reduction. Serve warm.