Finely grate the lemons, avoiding the bitter white pith, until you have 1 tablespoon plus 1 1/2 teaspoons of zest.
Juice the fruits, straining the seeds until you have 1/3 cup plus 1 tablespoon juice (90 grams).
Whisk together the honey and oil in a medium microwave-safe bowl, then whisk in the egg, lemon zest, juice and salt until smooth.
Don't worry if the honey hasn't completely dissolved at this point.
Set your microwave at 50% power.
Heat the mixture for 1 minute, then stop to whisk and scrape around the sides of the bowl.
Repeat, then continue heating and checking every 30 seconds, whisking and scraping in between; the mixture will foam and gradually thicken.
The custard is done once it coats the back of a wooden spoon and a path remains when you slide your finger across (this should take about 3 minutes total, depending on the power of your microwave).
The temperature of the custard should register at least 170 degrees on an instant-read thermometer. (This is hot enough to cook the egg.)
Let cool in the bowl for about 15 minutes, whisking a few times.
Strain the curd through a fine-mesh strainer for a super-smooth silky spread.
Spoon the curd into an 8-ounce glass jar.
Chill, uncovered, until completely cool, then seal the lid.
To make a tart lemon curd, replace the lemons in the basic recipe above with 2 medium lemons (250 grams). Use 1 tablespoon more honey for a total of 3 tablespoons plus 1 1/2 teaspoons at the start (70 grams).
To make an orange curd, replace the lemons in the basic recipe above with 1 medium orange plus 1/2 small orange (250 grams).
Tips from the author:
This recipe can easily be doubled. A double batch will take a few minutes longer to cook, about 5 minutes, depending on the strength of your microwave.
If only a tablespoon of citrus juice is missing, just use water so you don't have to cut another fruit.
If your honey has hardened, warm it first at 50% power in the microwave for 10 to 20 seconds until liquid; allow to cool a bit before using.
To make tart lemon curd and an orange curd (sweeter than the lemon curd)
The curd can be refrigerated for up to 1 week, and frozen for at least 1 month; defrost in the refrigerator overnight.