Set the strainer over a medium bowl.
Whisk the eggs (or egg and yolks) in a small nonreactive saucepan to blend. Whisk in the lemon zest, juice, and sugar. Add the butter. Whisk over medium heat, reaching into the corners and scraping the sides and bottom of the pan, until the butter is melted and the mixture is thickened and beginning to simmer around the edges, then continue to whisk for about 10 seconds longer.
Remove from the heat and scrape into the strainer, pressing gently on the solids. Scrape any lemon curd clinging to the underside of the strainer into the bowl.
Refrigerate until chilled before using. Lemon curd keeps in a covered container in the refrigerator for up to 1 week.