In a medium pot combine the rhubarb, sugar, lemon juice and water. Simmer on low for 10-15 minutes, stirring occasionally, until the rhubarb is very tender. Remove from heat and cool completely.
Using a mixer, whip the heavy cream until soft peaks form. Fold in the sweetened condensed milk and granola clusters.
Swirl in the cooled rhubarb mixture, making sure to not fully mix it, to leave some nice rhubarb swirls. Save a few spoonfuls of rhubarb to swirl on the top if desired.
Use a spatula to transfer the mixture to a loaf pan. Swirl the reserved spoonfuls of rhubarb on top.