PEEL the zest off the lemons in thin strips with a vegetable peeler, then coarse-chop the zest; it will be strained out of the curd later. Squeeze the lemons and strain the juice; weigh or measure the correct quantity of lemon juice. Caution: Make sure that no white pith gets into the curd mixture, either from peeling the zest, (cut any white pith off the backs of the peeled strips with a pairing knife if necessary), or from juicing the lemons (don't squeeze too close to the pith). Either of these things could cause the curd to be bitter.

COMBINE the zest, juice, sugar and butter in a heavy saucepan; or, if making the large recipe, use a Dutch oven or rondeau. Bring to a boil over medium heat.

WHISK the yolks in a bowl until liquified. Temper half of the boiling liquid into the yolks, whisking constantly. Pour the yolk mixture back into the pot and insert a probe thermometer into the liquid. STIR constantly in a "figure-8" motion with a wooden spoon or heat-resistant rubber spatula (red handle), going often into the corners of the pot. Continue cooking slowly over medium-low heat until the curd thickens to a firm-set, about 10 minutes overall. During this time, turn the heat on and off as necessary, stirring constantly, to prevent the curd from coming to a boil and to maintain a temperature of about 178°F (81°C). If you don't have a thermometer, cook until you can run your finger through the curd on the back of the spoon and a mark remains; this is called the "nappe" stage.

TO REPEAT: Cook to between 178° (81°C) and no higher than 180° F (82°C), which is the maximum temperature to prevent the curd from coming to a boil. Must cook gently, stirring constantly in a "figure-8" motion. NEVER BOIL.

REMOVE from heat and Immediately strain through a chinois into bain marie (or through a strainer into a large bowl). Press plastic wrap directly onto the surface of the curd and chill it in an ice bath.

BLOOD ORANGE CURD: Substitute zest of 4 Blood Oranges: Use 140g blood orange juice and 30g lemon juice. Reduce sugar to 120g.

ORANGE CURD: Substitute zest of 2 large Navel oranges: Use 120g orange juice and 50g lemon juice. Reduce sugar to 120g.

KEY LIME CURD: Substitute zest of 6-8 limes, depending on size: Proceed as for Lemon Curd, substituting lime juice for the lemon juice. For lime juice, use half fresh lime juice and half Nellie & Joe's bottled juice.

PASSION CURD: Substitute frozen passion fruit purée: Proceed as for Lemon Curd, using the purée in place of lemon juice and omitting the zest.