Cook the rhubarb: In a medium saucepan, combine chopped rhubarb and water. Cook over medium heat until the rhubarb breaks down and becomes soft, about 10 minutes.

Blend and strain: Transfer the mixture to a blender and purée until smooth. Strain through a fine mesh sieve to remove any fibers. You should have about 1 cup of rhubarb purée.

Whisk the eggs and sugar: In a medium heatproof bowl, whisk together the sugar, egg yolks, and whole egg until well combined.

Cook the curd: Slowly whisk the warm rhubarb purée into the egg mixture to temper it. Pour the mixture back into a clean saucepan. Cook over low heat, stirring constantly, until thickened and coats the back of a spoon, about 8–10 minutes. Do not let it boil.

Finish the curd: Remove from heat and stir in the butter, lemon juice, and salt until smooth.

Cool and store: Transfer the curd to a clean jar or bowl. Press plastic wrap directly on the surface to prevent a skin from forming. Chill until set. Store in the fridge for up to 1 week.