In a small saucepan over medium heat combine the halved and pitted ripe plums (keep the skins), water, lemon juice, sugar and a pinch of salt, cook 15min until the plums are soft and the syrup gets tinted deep red from the plum skins. Take off heat, cool slightly. Purée the mixture in a blender, then strain through a fine mesh sieve for a glossy smooth texture. Chill in the fridge overnight.
Freeze in your ice cream maker according to the manufacturer's instructions, store in the freezer for at least 4 hours until set or spoon right away.