Rinse the rhubarb, trim the ends, and slice it into 1-inch pieces. Place the rhubarb in a heavy-bottomed pot along with the sugar. Add 2 tablespoons of water and the lemon juice, then stir to combine.

Heat the mixture, stirring constantly, until the rhubarb starts to release juice and comes to a boil. Let it gently boil for about 15-20 minutes until the rhubarb is soft and mostly broken down.

Puree the mixture in a blender or food processor until smooth. Be cautious when blending hot foods to prevent splattering. Ensure the top is securely on and vented.

Return the puree to the stove, bring it to a boil, then reduce the heat and let it simmer until thickened, about 20 minutes. Stir frequently to prevent scorching. The butter will darken and become glossier as it nears completion. To test readiness, drag a spoon across the pan; the butter should not fill in the space.

Spoon the finished butter into a glass jar. Allow it to cool, then refrigerate after capping. The butter will thicken further as it chills. You should yield approximately one and a half cups.