Prepare Ingredients: Wash all nuts, seeds, and rose petals thoroughly. Soak almonds, cashews, pistachios, fennel seeds, mixed seeds, rose petals, and poppy seeds in water for 2 to 3 hours. Wash black pepper separately but do not soak.
Grind Ingredients: Drain soaked ingredients. Using a mortar and pestle or mixer, grind almonds, cashews, pistachios, fennel seeds, mixed seeds, rose petals, black pepper, and poppy seeds together gradually. Add some of the soaking water as needed to aid grinding.
Add Cardamom: Add freshly ground green cardamom seeds to the mixture and continue pounding or grinding for another 8 to 10 minutes until the mixture is smooth and no crunchiness remains.
Sweeten the Mixture: Add ground stringy candy (mishri) to the mixture for sweetness. Adjust sweetness to taste by adding more candy or sugar if desired.
Mix with Water and Milk: Add about one glass of water to the mixture and stir well. Then add approximately one kilogram of boiled, skimmed milk (diluted if full cream) and mix thoroughly.
Strain the Drink: Filter the mixture through a fine mulberry cloth or cheesecloth to remove solids. Alternatively, you can skip filtering and drink it as is.
Serve: Pour the strained Shirdai into glasses, add ice cubes, and garnish with rose petals. Serve chilled.