Hello, again Marlene.
I'm sorry you're having this trouble. Yes, the lemon curd should start to thicken more, after it's cooked; it's possible you did not cook it long enough, but you don't want to risk burning it, either. If you already took it off the stove, yes, you can make 'slurry' (not a paste) with cornstarch and water and add it in, as you continue stirring, or you can add some gelatin to thicken it. See instructions, below:
To use cornstarch as your thickener, heat the sweetened lemon mixture until it simmers. Whisk the cornstarch (2 TBSP into a small amount 1/3 cup of cold water, and stir it briskly into your filling. Within a minute or two, it will thicken. Cornstarch fillings can be baked, but the acidity will make them break down after a day or two.
I hope this helped and you can salvage your thin lemon curd for your tarts!
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