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.
In a blender, purée chilled milk with yoghurt, rhubarb purée and ½ of raspberry purée with ice. Divide remaining raspberry purée between 4 tall glasses. Top up with lassi mixture. Garnish with toasted almond slices.