Combine lemon juice, eggs, butter, sugar, and lemon zest in a 2-quart saucepan; cook over medium-low heat, stirring constantly, until curd is thick and just beginning to boil, about 8 minutes.

Remove curd from heat and cool to room temperature, stirring occasionally to keep the surface soft, about 30 minutes. Pour curd into clean jars, seal, and store in the refrigerator.