Combine the chopped rhubarb and granulated sugar in a large, heavy-bottomed pot or Dutch oven. Stir gently to combine. Let the mixture sit, uncovered, at room temperature for at least 30 minutes (or up to 2 hours in the refrigerator) to allow the sugar to draw out the juices from the rhubarb.

Place the pot over medium-high heat. Bring the mixture to a rolling boil, stirring frequently to prevent scorching on the bottom. Adjust the heat as necessary to maintain a vigorous boil.

Once boiling, add the fresh lemon juice and the tablespoon of butter (if using). The butter helps to break down any surface foam that forms during cooking.

Continue to cook rapidly, stirring very often, until the jam reaches the setting point. This usually takes 20 to 30 minutes after the rolling boil begins. Test for setting using a chilled plate (see notes).

Remove the pot from the heat. Skim off any lingering froth from the surface with a spoon. Let the jam rest for 5 minutes before carefully ladling the hot jam into sterilized canning jars, leaving 1/4 inch headspace.

Seal the jars according to your preferred method (water bath canning for long-term shelf stability, or refrigerate if consuming within 3 weeks). Cool completely on a rack before storing or serving.