Preheat oven to 350 degrees. In a medium bowl, using the back of a wooden spoon, grind salt with lime zest. Add dark-brown sugar, red-pepper flakes, and egg white and whisk together until frothy. Add cashews and toss to coat.
Spread cashew mixture in a single layer onto a parchment-lined rimmed baking sheet. Bake until golden, 20 minutes. Let cool completely.