Stir together the corn starch and the soy milk for the slurry until no lumps of starch remain.
Combine the remaining soy milk, lemon juice and zest, sugar, turmeric (if using) and the cornstarch slurry in a saucepan - everything except the vegan butter. Don't worry if the soy milk curdles at this point when it gets in contact with the lemon juice - The lemon curd will be completely smooth later!
Whisk frequently over medium-high heat while you bring the mix to a simmer.
Once the liquid reaches boiling temperature and suddenly starts to thicken, reduce the heat to medium and simmer for 1 minute, still whisking very frequently.
Take the saucepan off the heat and whisk in the vegan butter until completely melted and well combined.
Pour the hot lemon curd into jars and let it cool down.The vegan lemon curd will continue to thicken as it cools down in the jars. If it firms up too much, just give it a good stir to bring it back to a smooth and creamy consistency.
Notes
Lemon curd should be stored in the fridge. It will keep for up to 1 week. For ideas on how to use lemon curd and tips on filling a cake, see the post.