Prepare the simple syrup: Combine the sugar and water in a small saucepan. Heat over medium heat, stirring constantly until the sugar is completely dissolved. Do not boil vigorously. Remove from heat and let cool to room temperature.

Prepare the nectarines: Slice the peeled and pitted white nectarines into chunks. If the nectarines seem pale, lightly toss them with 1 teaspoon of lemon juice to prevent browning.

Blend the mixture: Transfer the cooled simple syrup, nectarine chunks, 1 tablespoon of fresh lemon juice, and vanilla extract (if using) into a high-speed blender. Blend until completely smooth and pureed.

Chill the base: Taste the mixture and adjust sweetness or tartness if necessary. Strain the mixture through a fine-mesh sieve into an airtight container to remove any remaining fibers. Chill the mixture in the refrigerator for at least 4 hours, or until very cold (preferably overnight).

Churn the sorbet: Pour the chilled nectarine base into your ice cream maker and churn according to the manufacturer's directions (usually 20 to 30 minutes) until it reaches a soft-serve consistency.

Harden and serve: Transfer the churned sorbet to a freezer-safe container. Freeze for an additional 2 to 4 hours to firm up. Allow the sorbet to sit at room temperature for 5-10 minutes before scooping and serving.