In a saucepan, cook the fresh rhubarb with 1/2 cup sugar over medium heat until very soft, about 8 minutes, stirring occasionally and watching for the fruit to break down.

Blend the cooked rhubarb until smooth, then return the puree to the same saucepan and stir to fully combine.

Whisk the remaining sugar, egg yolks, and lemon juice into the rhubarb puree until smooth and evenly colored.

Cook over medium-low heat, stirring constantly, until the mixture thickens and coats the back of a spoon, about 6-8 minutes, and it should look glossy and slightly thicker with each minute.

Remove from the heat and whisk in the butter cubes until completely melted and smooth, ensuring no visible streaks remain.

Strain through a fine mesh sieve for extra smoothness if desired, using a spatula to press the curd through and leaving any thicker bits behind.

Pour the rhubarb curd into clean jars, cover, and refrigerate for at least 2 hours until set and spoonable, with the texture turning silkier as it chills.

Store the jars in the refrigerator for up to 2 weeks, keeping them sealed between uses.