Question: My lemon curd often turns out lumpy, even when I stir constantly. What am I doing wrong?
Answer: You might have the heat turned up too high.
There is a trick to making a smooth, silky lemon curd, a sort of custard that is used to fill tarts or cakes: heat the mixture to just the point when the eggs bind and thicken it, but don't let it boil, which will scramble the eggs into tough little lumps. The ideal temperature is 180 to 200 degrees; if you don't have a thermometer, you can recognize this point when the mixture has the texture of a rich cream sauce.
Another way to guard against lumps is to stir the eggs and sugar together thoroughly before adding the lemon juice (citric acid, or any other acid, will curdle eggs unless they are protected by the addition of sugar).
You can also try cooking the mixture in a double boiler over simmering water, or in a thick, heavy saucepan over very low heat. Whatever method you use, be sure to stir the curd constantly with a wooden spoon or spatula, and make sure to stir around the bottom and edges of the pan.
If the curd accidentally comes to a boil, all is not lost; you can remove the lumps by immediately passing the curd through a fine sieve. Actually, it's generally a good idea to strain lemon curd and other custards after cooking, since this ensures a lumpless cream (if the recipe calls for lemon zest, add it after straining).
In many professional kitchens, custards are purposely brought to a boil, then strained to remove the lumps. While this works as a shortcut, the result will never be as satiny as a slowly cooked lemon curd.
Lemon curd keeps for about a week in the refrigerator. Besides filling cakes and tarts, it can also be used as a sprightly topping for fresh fruit or berries and folded into whipped cream to make an instant lemon mousse.