Soak the rice in water for half an hour. Drain water and pulse in a grinder till you get a grainy consistency. The grains should feel gritty between your fingers and should be slightly bigger than semolina.
Bring the milk to a gentle simmer, and add the rice and saffron while whisking continuously. This will help avoid lumps.
Bring this to a boil while whisking, and then lower the flame to a simmer. Keep stirring the mixture every couple of minutes.
After about 20 minutes the mixture will start to thicken. Simmer for another 20 -25 minutes till it thickens considerably and the rice is completely cooked.
Add sugar and mix till it dissolves. Switch off the flame and let the mixture cool. Once its cooled completely, transfer to a mixer-grinder and pulse for exactly 5 seconds. This helps give the phirni a creaminess and remove any lumps.
Mix in the rose petals, and transfer into serving bowls. Sprinkle with chopped nuts and rose petals. Cool in the refrigerator for at least 2-3 hours or overnight. Serve cold.