Fill a large pot with water and bring it to a rolling boil. While the water heats, thoroughly wash your oranges, scrubbing vigorously to remove any surface wax. Slice off the stem and blossom ends of the oranges. Slice the oranges into uniform, approximately 1/8 to 1/4 inch thick rounds. Once the water is boiling, carefully add the orange slices. Boil for exactly 5 minutes.

Drain the slices immediately and transfer them to an ice bath or rinse them under cold running water to stop the cooking process and keep the slices vibrant. For reduced bitterness, repeat the blanching step one more time (a second 5-minute boil in fresh water).

In a wide, heavy-bottomed pot or deep skillet, combine the 3 cups of water and 3 cups of granulated sugar. Bring this mixture to a boil over medium-high heat, stirring until the sugar is completely dissolved and the liquid is clear. If using, add the cinnamon sticks now.

Reduce the heat to maintain a very gentle simmer. Add the blanched orange slices in a single layer (or close to it). Ensure the syrup completely covers the oranges. Simmer very gently, uncovered, for 60 to 90 minutes. The goal is low and slow infusion; high heat will caramelize the sugar too fast.

Turn off the heat and allow the orange slices to cool completely in the syrup, preferably overnight, or for a minimum of 8 hours.

Preheat your oven to its lowest setting (usually 175°F to 200°F). Line two baking sheets with parchment paper or silicone mats. Carefully remove the slices from the syrup, letting excess drip back (save the syrup!). Place slices on the baking sheets. Bake for 30 to 60 minutes, turning halfway through.

Roll the slightly tacky slices in the reserved 1/2 cup of granulated sugar. Work quickly so the slices don't dry out entirely before being coated.

Transfer the coated slices to a wire rack and allow them to dry completely until firm to the touch.