In a medium saucepan, combine the rhubarb, lemon juice and sugar over medium heat. Cook for 8-10 minutes, stirring occasionally, until thickened and the sugar dissolves. It should coat the back of a spoon and you should be able to see the bottom of the pot when scraped. Remove from heat and set aside to cool.
In a large bowl, whisk together the buttermilk, cream cheese, condensed milk, vanilla, and salt until thick and smooth. Some lumps are ok.
Place the whipping cream into a blender or food processor and blend for 1-2 minutes until stiff peaks form. Spoon out the whipped cream into a large bowl. Alternatively, you can use an electric mixer or stand mixer to whip the cream.
Fold in the cream cheese mixture into the whipped cream until just combined. Do not overmix.
Pour half of the cream mixture into a chilled glass dish or loaf pan. Drizzle in half of the rhubarb mixture and swirl into patterns with a chopstick or butter knife.=
Gently pour in the remaining cream mixture and top with the rhubarb puree. Repeat the swirling pattern.
Cover the loaf pan with plastic wrap, pressing down so the plastic wrap is lying flat against the top of the ice cream (this prevents ice crystals).
Freeze the ice cream for at least 4 hours or overnight. To serve, remove from the freezer and let it stand for 5 minutes before scooping.
To scoop ice cream, place a spoon or ice cream scoop in a cup of hot water for 2-3 seconds to make scooping easier. Alternatively, allow the ice cream to thaw a further 5 minutes to soften.