Using a micro-wave oven, mix the peeled and rather thin-cut pieces of rhubarb with the sugar(s), and the vanilla, the pinch of salt and the sliced pieces of lemongrass in a microwave-safe bowl.
Micro-wave the mix for about 6-9 minutes, eventually whisking once or twice during the process, until the rhubarb has turned into a smooth purée.
Take out from the microwave and let the purée cool down. Then, preferably, let the purée chill in the fridge for at least a couple of hours.
Whip the cream rather stiff, and blend in the yoghurt.
Remove the sliced pieces of lemongrass from the (chilled) rhubarb- purée, and add the purée to the cream-yoghurt blend.
Churn in your ice cream machine according to instructions, or - without an ice cream machine - still-freeze the ice cream using your freezer (for best results, see this site's post on how to make ice cream without an ice cream machine).