Prepare the Baking Sheet:

Line a baking sheet with parchment paper or a silicone baking mat and set it aside.

Heat the Sugar Mixture:

In a large nonstick skillet or saucepan, combine the sugar, water, cinnamon (if using), and salt. Heat over medium heat, stirring occasionally, until the sugar dissolves.

Cook the Walnuts:

Add the walnuts to the skillet and stir to coat them in the sugar mixture. Continue cooking, stirring frequently, until the liquid thickens and turns into a syrup that coats the walnuts evenly.

Caramelize the Walnuts:

Keep stirring as the sugar begins to crystallize and then melts into a golden caramel. Be careful not to burn the sugar; adjust the heat if necessary.

Transfer to Baking Sheet:

Quickly spread the candied walnuts onto the prepared baking sheet, separating them with a fork if clumped together.

Cool Completely:

Allow the walnuts to cool completely on the baking sheet. Once cool, they will harden and become crunchy.