3 large eggs or 1 large egg plus 3 large egg yolks
Zest of 1 medium lemon
1/2 cup strained fresh lemon juice (from about 3 medium lemons)
1/2 cup sugar
6 tablespoons unsalted butter, cut into chunks

Put a medium mesh strainer over a clean bowl near the stove. Put all of the ingredients from your favorite recipe (or mine, below) into a heavy non-reactive saucepan, adding the butter last, and whisk briskly over medium heat. After the butter melts, the mixture will thicken and start bubble at the edges; continue to cook and whisk (sweeping the sides and corners of the pot) for another 15 to 20 seconds. Strain into the clean bowl, pressing gently on any solids. Let cool and refrigerate.