Wash the rhubarb stalks and cut them into about ¾" pieces. If the stalks are fat, cut them in half lengthwise first.
Place the rhubarb and ¼ cup of water in a saucepan with a tight-fitting lid. Place over low heat until the rhubarb starts releasing its liquid, stirring often. When there is a bit of visible liquid, add the sugar, stir well, cover and place over low heat until the sugar dissolves. Raise the heat to medium and bring to a low boil, keeping the lid on.
Cook until the rhubarb is very tender. Remove from the heat and cool to lukewarm. Add the gelatin to ¼ cup of cold water, stirring well. Set aside.
When the rhubarb is cooled, place it in the bowl of the processor. Process until smooth. Heat the gelatin in the microwave for about 20 seconds to liquefy. Add to processor and process briefly. Cool completely.
Beat the cream until soft peaks form. Do not overbeat. Fold into the rhubarb and pour it into the prepared shell. Refrigerate until set. I leave mine in the refrigerator overnight.
To release the tart, warm the removable rim of the pan with a hair blower set on high heat, held about ¼ inch from the tin. Circle the entire tin. Place it on top of a fat can and gently pull down the sides. If it doesn’t release, re-warm the edge. Do not overheat, or you can melt the sides of the tart.
To move the tart, release the bottom with a metal spatula. Place two pancake turners, one on each side of the tart, and lift it onto the serving plate. Or, if it is loose enough, just slide it off the bottom onto a serving plate. To make ahead, freeze, well wrapped in the pan, for up to a month. To use, thaw overnight in the refrigerator.