Preheat your oven to 250°F and lightly grease a large baking sheet; avoid using parchment paper or foil as the nuts should roast directly on the pan. In a small bowl, whisk together the sugar, cinnamon, and salt and set aside. In a large mixing bowl, beat the egg white with the vanilla extract using a whisk until the mixture becomes frothy, which should take around one minute. Add the walnuts into this frothy mixture and stir well to ensure each nut is fully coated. Once the walnuts are evenly coated, gradually add the dry sugar mixture and stir until the coating is uniformly distributed; be sure to scrape the sides and bottom of the bowl to incorporate all the cinnamon-sugar. Spread the walnuts in a single, even layer on the prepared baking sheet. Place the pan in the oven and bake for 1 hour, stirring thoroughly every 15 minutes. Even if the walnuts appear finished before time, bake for the entire hour to achieve the optimal texture. Once done, remove from the oven, give them one last stir, and allow them to cool on the baking sheet or enjoy warm.