Cut the rhubarb into 1/2-inch pieces and place in a bowl with the sweetener. Stir to combine, then cover and let sit overnight at room temperature. This maceration step is crucial—it draws out the rhubarb's natural juices and allows the sweetener to dissolve, creating a syrup base that will help the jam set properly. I find this overnight step essential for achieving the right consistency without needing additional pectin.

Pour the rhubarb mixture (fruit, accumulated liquid, and sweetener) into a heavy-bottomed saucepan or jam pan. Add the freshly squeezed lemon juice and a pinch of salt. The lemon juice activates pectin naturally present in rhubarb, helping the jam achieve the proper gel, while the salt enhances the fruit flavor without making it taste salty. Bring to a rolling boil over medium-high heat, stirring occasionally to ensure even cooking.

Once boiling, cover the pan and reduce heat to medium-low. Simmer gently for 15 minutes, stirring occasionally. The rhubarb will break down and the liquid will reduce and thicken. I like to check the consistency by placing a small spoonful on a chilled plate—if it wrinkles slightly when pushed, it's ready. The jam will continue to thicken as it cools, so don't over-cook it.

Remove from heat and add one drop of red food dye if desired, stirring well to distribute the color evenly throughout the jam. Pour the jam into sterilized jars and let cool to room temperature on the counter. As it cools, it will continue to set and reach its final gel consistency.