Cut the fresh rhubarb into 1/2-inch pieces and combine with sugar in a large, heavy-bottomed pot or large bowl. Stir well to coat the rhubarb evenly with sugar. Let this mixture sit at room temperature overnight (or at least 8 hours). This maceration process breaks down the rhubarb's cell walls and draws out the natural juices, which will help the jam set properly and develop deeper flavor. The rhubarb will release a significant amount of liquid during this time.

Transfer the macerated rhubarb and sugar mixture to a large pot over medium-high heat. Bring to a rolling boil and cook for exactly 12 minutes, stirring occasionally to prevent sticking on the bottom. I like to add the lemon juice at the start of cooking—it brightens the flavor and helps activate the pectin for better setting. You'll notice the mixture will reduce significantly and the rhubarb will soften and break down. At the 12-minute mark, the jam should reach the proper consistency and gel point.

Remove the pot from heat immediately after the 12 minutes are complete. Add the strawberry gelatin mix all at once and stir continuously for 1-2 minutes until the gelatin is completely dissolved and no lumps remain. The residual heat from the hot jam will dissolve the gelatin evenly. I've found that this combination of natural jam-setting plus gelatin creates a more reliable set, especially if your rhubarb was lower in natural pectin. Let the jam cool for a few minutes before jarring.

Pour the jam into clean jars while still warm, leaving about 1/4-inch headspace at the top. Allow the jars to cool to room temperature, then cover tightly and refrigerate. The jam will continue to set as it cools. Store in the refrigerator for up to 3-4 weeks, or freeze for longer storage.