Add the chopped pistachios to a medium saucepan over medium-low heat. Toast, stirring frequently, until fragrant and lightly golden, about 4–5 minutes.
Add the water, granulated sugar, honey, and flaky sea salt.
Bring the mixture to a gentle simmer, stirring until the sugar has completely dissolved.
Simmer for 5–6 minutes.
Remove the saucepan from the heat and allow the pistachios to steep in the syrup for 20–30 minutes.
Strain the syrup through a fine mesh sieve into a heat-safe measuring cup or bowl, pressing gently on the pistachios to extract as much syrup as possible.
Let the syrup cool completely before transferring it to a clean glass bottle or jar.
Refrigerate until ready to use.