Combine rhubarb, ¼ cup honey, and lemon juice in a saucepan over medium heat. Cook until liquid starts to bubbly around the edges then reduce heat to medium-low, cover and cook 10–15 minutes until rhubarb is completely soft, stirring occasionally. Uncover and cook, stirring constantly, 1 minute. Transfer rhubarb mixture to a blender and puree; return rhubarb puree to saucepan.  

Whisk together remaining ¼ cup honey, egg yolks and egg until thoroughly combined; whisk into rhubarb mixture. Cook curd mixture over medium-low heat, stirring constantly with rubber spatula, until mixture thickens slightly and registers 168°F on an instant-read thermometer, 5–8 minutes.

Off heat, whisk in butter until melted. Strain rhubarb curd through fine-mesh strainer into bowl.