Combine the chopped rhubarb, sugar, and lemon juice in a large, heavy-bottomed pot. Stir well to coat the rhubarb with sugar and allow it to sit for at least 30 minutes, or up to a few hours, to macerate.

Place the pot over medium heat and bring the mixture to a boil, stirring occasionally. Once boiling, reduce the heat to low and simmer.

Continue to simmer the jam, stirring frequently to prevent sticking, until it thickens and reaches a gel-like consistency; this typically takes 45-60 minutes.

To test for doneness, place a small spoon of jam on a chilled plate, wait 30 seconds, then push the jam with your finger; if it wrinkles, it's ready. Alternatively, use a candy thermometer to reach 220°F (104°C).

Remove the jam from the heat and carefully ladle it into hot, sterilized jars, leaving 1/4 inch headspace. Wipe the rims clean with a damp cloth.

Seal the jars with new lids and process them in a boiling water bath for 10 minutes to ensure a proper seal. Let the jars cool completely on a wire rack before storing.