For the sorbet: Sorbet: In a blender, purée pineapple, water and 3⁄4 cup granulated sugar until smooth. Chill mixture until cold, about 30 minutes, then transfer to an ice cream maker and freeze for 25 minutes. Beat egg whites with remaining 2 tablepoons sugar; add to mixture in ice cream maker and freeze 5 minutes more, then transfer to an airtight container and put in freezer to harden, at least 90 minutes.

For the pesto: In a small saucepan, combine water and Demerara sugar. Heat over medium-high heat, stirring, until sugar is dissolved, about 2 minutes. Remove syrup from heat.

Using a sharp paring knife, remove 2 long wide strips zest from lemon, avoiding white pith. Bring a second small saucepan of water to boil. Add zest and cook until softened, about 2 minutes; drain.

Squeeze 2 tablespoons juice from lemon. Combine lemon juice and peel, syrup, celery leaves, pine nuts and pistachios in a food processor; pulse until smooth.

Serve sorbet with pesto.