Add lemon juice, water, sugar, cornstarch, cashew butter and lemon zest to a small saucepan and whisk to combine.
Heat over medium-high, whisking constantly, until everything melts and it comes to a gentle boil.
Whisk in the turmeric, if desired.
Cook on low-medium until thick, whisking the entire time. This takes about 5 minutes.
Once thick, remove from the heat.
Allow it to cool for 5 minutes before adding it to a baked tart or pie shell. Alternatively, pour it into a jar or container and let it cool uncovered. Once cooled, refrigerate it. Let it set for at least 3 hours but ideally overnight. It will thicken further as it cools in the refrigerator.