Place the chopped rhubarb and granulated sugar in a medium saucepan. Cook over medium-low heat, stirring occasionally, until the rhubarb has softened and released its juices, about 8-10 minutes. Mash the rhubarb gently with a fork or potato masher as it cooks.

Strain the cooked rhubarb mixture through a fine-mesh sieve set over a clean saucepan. Press gently on the solids to extract as much liquid as possible. Discard the solids.

Whisk the lightly beaten eggs into the rhubarb liquid in the saucepan. Add the lemon zest, if using.

Cook the mixture over low heat, stirring constantly with a wooden spoon or whisk, until it thickens enough to coat the back of the spoon. Do not let it boil, or the eggs will scramble. This will take about 5-8 minutes.

Remove the saucepan from the heat. Add the cold butter cubes, one by one, whisking until each cube is melted and incorporated before adding the next. Continue whisking until the curd is smooth and glossy.

Pour the curd into a clean jar or bowl. Press a piece of plastic wrap directly onto the surface of the curd to prevent a skin from forming. Let it cool at room temperature, then refrigerate until fully chilled and set, at least 2 hours.