In a saucepan off of the heat, whisk together the milk, cornstarch, sugar, turmeric until well combined.
Set the sauce pan over low heat and bring to a simmer while whisking continuously. Let boil gently for at least 1 minute to cook the starch.
Keep stirring fast and vigorously to avoid any lumps. Stop when you reach your desired consistency.
Add your flavouring ingredient (lemon zest, vanilla extract, or both). If you use lemon zest make sure to only use the yellow part of the lemon and not the white flesh under the skin. That part is bitter.
Let the custard cool at room temperature. Whisk occasionally to prevent the formation of skin on the surface. If you want to cool it down faster, you can submerge the pan in cold water while whisking until the custard is cold.