Zest one of the lemons directly into a large microwave-safe bowl. Juice all three lemons into a measuring cup. You need approximately ½ cup of lemon juice. Add the juice to the bowl with the zest.
Add the eggs, sugar, and salt to the bowl. Whisk until smooth, then let the mixture sit for 10 minutes to help the sugar begin to dissolve.
After 10 minutes, whisk the mixture vigorously. Break the butter into small pieces and add them to the bowl.
Microwave the mixture on full power for 1 minute. Remove the bowl from the microwave and check the temperature using a digital thermometer. The target temperature is 170ºF (76ºC).
Whisk the mixture again to help melt the butter and smooth out the texture. Return the bowl to the microwave and cook for another 1 minute.
Remove the bowl, whisk thoroughly, and check the temperature again. Continue microwaving in 1-minute intervals, whisking after each, until the curd reaches approximately 150ºF (65ºC). Then, reduce the microwave time to 30-second intervals, whisking in between, until the curd reaches 170ºF (76ºC). It should be thick and similar in consistency to honey.
Give the lemon curd a final whisk to ensure it is smooth. Spoon it into clean jars or containers, leaving the lids slightly ajar for 2 hours to prevent condensation from dripping onto the surface. Cool the curd completely before securing the lids and storing it in the fridge.
Notes
Zesting the lemon directly over the bowl helps release the oils and captures more flavour than zesting onto a cutting board or plate.
If seeds or pulp are in the lemon juice, strain it before adding it to the bowl.
Microwave ovens vary in power, so keep a close eye on the temperature after the second minute.
The curd will continue to thicken as it cools, so don’t worry if it seems a little loose when it reaches 170ºF (76ºC).
Storage
Let the lemon curd cool completely before sealing the jars to avoid trapping steam. Store in the fridge for up to 2 weeks.