First soak the sago in a bowl with enough water for atleast 4-5 hours or overnight.
On the next day, strain the sago and keep aside.
Then peel the mango, chop roughly and then blend into a smooth puree type without adding any water. Keep aside.
Now, heat milk in a deep pan and allow to boil.
After the first boil, simmer it and add the sugar, soaked sago, cardamom powder, salt.
Meanwhile in a small bowl take mango custard powder along with 2-3 tbsp of milk and mix well without any lumps. Keep aside.
When sugar melts completely, add the custard mixture and using a spatula / hand whisk mix everything quickly to avoid any lumps formation.
Soon it will start to thicken, keep mixing using a spatula / hand whisk.
Keep stirring and after some time, the sago balls look transparent and the whole liquid mixture starts to nicely coat the spatula. It means the sago kheer is nicely cooked by now.
Turn off the heat and allow the kheer to cool down a bit.
Then add the mango puree and mix evenly.
Transfer to a container, cover it and keep in refrigerator till serving.
At the time of serving, pour into desired bowl and sprinkle some chopped nuts on top of the chilled mango sago kheer. Serve !