Wash and soak the rice for 1 hour, covered in enough water.
Drain the rice and grind it with the coconut in a food processor (using a little water, not too much) till you get a thick, slightly grainy, batter-like paste (it should not be completely smooth).
Transfer this mixture to a large saucepan, add 500 ml water and ground cardamom, and cook on a low-medium flame for 10 mins (stirring constantly), till the phirni is cooked to a creamy, gruel-like consistency. If your phirni thickens too much, thin it out with a little water.
Turn off the heat, stir in the sugar and the ground nutmeg, stir well and set aside to cool.
Meanwhile, peel, dice and puree the mango with 2 tsp lemon juice (to help retain the bright colour).
Once the phirni has cooled completely, stir in the mango and mix well. Toss in the pistachios, chill for at least 2 hours and serve with fresh mango.