Bring the lemon juice - barring 1 tbsp - to the boil in a medium saucepan.
Place 1 tbsp lemon juice, cornflour, sugar, and turmeric, if using, into a blender, and blend for 30 seconds or so, until smooth.
Once the lemon juice is boiling, tip the contents of the blender into the pan, and whisk well.
Bring back to the boil, whisking all the time, to ensure there are no lumps. Reduce the heat to medium, and continue to cook for around a minute, to dissolve the sugar.
Reduce the heat to low, and simmer - stirring occasionally - for 3-5 mins, to make sure the cornflour is properly cooked in, and the lemon curd is smooth and silky.
Remove from the heat, and beat in the margarine.
Pour the lemon curd into a sterilised jar, and seal.
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