In a small saucepan, combine the water, granulated sugar, and lemon zest. Heat over medium, stirring, until the sugar dissolves and the syrup turns clear, 2–3 minutes after it reaches a bare simmer. Remove from heat, cover, and let steep for 10 minutes.
Strain the syrup through a fine-mesh sieve into a medium bowl to remove the aromatics. Cool to room temperature, 20–30 minutes (or set the bowl in an ice bath to cool in about 10 minutes).
Stir in the lemon juice until evenly combined.
Pour the mixture into a shallow metal baking pan and freeze for 30 minutes.
Scrape the icy edges into the center with a fork, then return to the freezer. Repeat every 30 minutes until the granita forms fine, fluffy crystals and is spoonable, 2–3 hours total.
Fluff once more with a fork. Serve immediately in chilled glasses, or transfer to an airtight container and freeze 10–15 minutes for a slightly firmer texture.