On a floured surface, roll out the thawed frozen puff pastry into a thin rectangle, about 1/8 inch thick. Roll tightly from one short end to the other, forming a cylinder, and slice into 12 equal pieces.

Place each piece cut-side down into a greased muffin tin and press down to distribute along the base and up the sides. Refrigerate for 30 minutes.

In a medium saucepan over low heat, whisk together sugar, flour, and a small scout of milk to create a smooth paste. Gradually add remaining milk, cinnamon stick, and lemon peel. Heat while whisking for 5-7 minutes until thickened, then let steep for 10 minutes.

Whisk egg yolks until pale yellow. Strain the warm milk mixture to remove cinnamon stick and lemon peel, then slowly pour into egg yolks while whisking continuously. Strain again through a fine sieve.

Preheat oven to 500°F (260°C). Pour custard into pastry shells, filling about 3/4 full. Bake for 12-15 minutes until tops are puffy and golden.

Cool the tarts in the tin for 5 minutes, then transfer to a wire rack. Serve warm, optionally dusted with powdered sugar and ground cinnamon.