In a blender or food processor, purée the raspberries, lime juice and salt until smooth. Strain through a fine-mesh sieve into a medium bowl, pressing with a rubber spatula to extract the juice; discard the seeds.

In a large saucepan, bring 1 inch of water to a simmer. In a large stainless steel bowl, combine the egg whites, granulated sugar and cream of tartar. Beat with an electric mixer on medium speed until foamy. Set the bowl over the simmering water and continue to beat on medium speed, until the mixture is glossy and thick, about 3 1/2 minutes. Increase the speed to high, and continue beating over the simmering water until very stiff and glossy, about 3 1/2 minutes. Remove from the heat and continue beating on medium speed until room temperature, 3 to 5 minutes.

Fold the raspberry puree into the egg whites until combined. Transfer to a freezer-safe container and freeze, covered, until solid, at least 6 hours.

To serve, let the sorbet stand at room temperature until softened slightly, about 10 minutes. Garnish with mint leaves, if desired.

Sorbet can be kept, covered and frozen, for up to 3 weeks.