Begin by washing your fruit (unless the blueberries are frozen), zest and juice the lemon, and set out the eggs to come to room temperature. Set a fine mesh sieve over a bowl.

Place the blueberries in the pot and add the sugar, salt, zest, and lemon juice over them. Cook the blueberries over medium low heat. Smash the blueberries and cook them until they’re very juicy, about 5-8 minutes.

Tempering the Eggs

In a separate bowl, beat the eggs until smooth. Ladle spoonfuls of the how blueberry mixture into the eggs while whisking to slowly warm them up. They will turn a strange greenish color, but that’s just the color of the blueberries mixing with the yellow of the yolks.

Add the warmed blueberries slowly back into the pot. Replace it on the heat and stir constantly until the mixture has thickened enough to coat the back of a spoon. Make sure not to overcook the curd, as that can make the egg curdle.

Remove the blueberry curd from the heat pour it through the sieve placed over a bowl. This removes the blueberry skins and also any tiny bits of eggs which may have cooked. Press the curd down with the back of a spoon to get as much through the sieve as possible.

While the curd is still warm, add the butter. Stir it in until the butter is melted and completely incorporated.