Simmer raspberries with sugar for 5 minutes, purée and strain. Chill until ready to use.
For rhubarb, simmer with sugar and water for 10 minutes until tender and purée. Chill until ready to use.
For lassi, heat ground almonds, cinnamon stick with milk to just below a simmer. Stir occasionally and keep at low heat for 10 minutes. Strain milk through a fine strainer lined with cheesecloth, twisting cheesecloth to get every bit of juice out of almonds. Chill completely.
Whisk together chilled milk, yoghurt, rhubarb puree and half of raspberry purée. Pour into an ice cream maker and churn according to manufacturer’s instruction. Right before you stop the ice cream maker, drizzle in remaining raspberry purée and let blend just 10 or 15 seconds for a lovely raspberry ripple! Spoon into a container and freeze until firm.