Preheat the oven to 325 degrees. In a large bowl, whisk together the sugar, 2 tbsp. lime juice and the fish sauce. Add the peanuts and toss well to coat.
Spread the nuts in a single layer on 2 parchment-lined baking sheets. Bake, stirring occasionally, until fragrant and golden-brown, about 20 minutes.
Let the nuts cool for 10 minutes, then transfer to a serving bowl. Toss with the lime zest; season with salt and pepper.