Fill a medium sized saucepan with 2 inches of water. Bring to a boil over high heat. Then turn the heat down to medium-low so the water is at a simmer.
Whisk together the lemon juice, lemon zest, sugar and eggs in a heatproof mixing bowl. Slice the butter into cubes and add to the egg mixture.
1 cup lemon juice, freshly squeezed (~5-6 lemons), 1 Tbsp. lemon zest (~from 2 lemons), 1 3/4 cups sugar, 4 large eggs, 1/2 cup salted butter
Place the bowl over the simmering water and get ready for a little arm workout! Whisk continuously for ~20-25 minutes or until the curd begins to thicken. The curd might look curdled from the butter at first, but it will smooth out as you continue to whisk.
This makes a big batch of lemon curd (2 cups), which is why it takes some time to thicken, but the time can also vary based on your stovetop burner. I like to check the temperature with a thermometer and you'll know when it's done when it reaches 170°F or can coat the back of a spoon.
Strain the curd through a fine mesh sieve into a glass bowl.
Cover with plastic wrap, pressing the plastic wrap down directly on the warm curd to help prevent a top layer of film from forming (this is called contact wrapping).
Allow the curd to cool at room temperature, then transfer to the fridge to cool completely, at least 4 hours or overnight. The curd thickens when it cools so allow it plenty of time to chill prior to using.
Refrigerate the curd in sealed glass jars for up to 1 week. Enjoy!!