For sorbet base: Combine simple syrup, water, lemon juice, and salt in a bowl. Stir well. (Base keeps, refrigerated in an airtight container, for up to 3 months.)
For sorbet: In a food processor or blender, puree pineapple, cilantro, and serrano. Transfer pineapple puree to a bowl and stir in Sorbet Base.
Process in an ice cream maker according to manufacturer’s instructions.
Scrape into an airtight storage container. Freeze for a minimum of 2 hours before serving.