1. Preheat oven to 400°F (200°C). Line two baking sheets with parchment paper.
2. Mix sugar and cinnamon in a small bowl until well combined.
3. On a lightly floured surface, roll puff pastry to approximately 12 x 14 inches (about ⅛-inch thick).
4. Sprinkle ⅓ of cinnamon-sugar mixture over the pastry, leaving a ¼-inch border along one long edge. Press sugar gently into the dough with a rolling pin.
5. Distribute diced rhubarb evenly over the sugared surface, avoiding the ¼-inch border. Sprinkle another ⅓ of cinnamon-sugar over the rhubarb.
6. Starting from the long edge without the border, fold the pastry inward in 1-inch increments until you reach the middle. Repeat from the opposite side until the two folded sides meet.
7. Fold one half over the other like closing a book. Gently press to seal.
8. Cut the folded pastry crosswise into ½-inch slices.
9. Place palmiers on prepared baking sheets 2 inches apart. Sprinkle with remaining cinnamon-sugar.
10. Bake for 12-15 minutes until deeply golden on the bottom, then flip each palmier and bake 3-5 minutes more until crisp and caramelized on both sides.
11. Cool on baking sheet for 2 minutes, then transfer to a wire rack to cool completely.

Notes:

• Keep puff pastry cold during preparation for best results.
• Pat rhubarb very dry to prevent soggy palmiers.
• Store in an airtight container at room temperature for up to 3 days.
• Unbaked palmiers can be frozen for up to 1 month and baked directly from frozen.

Nutrition (Per Palmier): Calories: 95, Fat: 5g, Carbohydrates: 11g, Protein: 1g, Fiber: 0.5g, Sugar: 6g