Cut the watermelon into 1-inch cubes, removing all seeds as you go, until you have about 4 cups of prepared fruit. Zest the lime directly into a blender, then add the watermelon cubes, 2 tablespoons of fresh lime juice, sugar, and kosher salt. Blend for 25 to 45 seconds until completely smooth—you want a uniform puree without any chunky bits, as this ensures even freezing and a silky texture in the final granita.

Pour the blended watermelon mixture through a fine-mesh strainer into a shallow baking dish (a 9x13 inch dish or similar works well), pressing gently on the solids to extract all the flavorful liquid; discard the pulpy residue. This straining step is crucial—it removes any remaining fiber that would create a grainy texture rather than the desired icy crystals. Transfer the dish to the freezer immediately.

Freeze for 1.5 to 2.5 hours total, scraping the mixture with a fork every 30 minutes. This repeated scraping is what transforms the mixture from a frozen block into light, icy granita crystals—without it, you'll end up with a solid popsicle-like texture. I like to scrape with firm, deliberate strokes from the edges toward the center, creating uniform ice crystals that give granita its signature texture. The granita is ready when it resembles wet, flavored snow with no large frozen chunks.

Once the granita reaches the desired slushy consistency, transfer it to serving bowls or glasses immediately—the icy texture will begin to consolidate if left sitting. Garnish each portion with a sprinkle of finely chopped fresh mint and a small pinch of lime zest if desired. I find the cold, bright flavors of the mint and any remaining lime brightness really elevate the simple watermelon, so don't skip the garnish.