In a bowl, add almonds and pistachios, soak in 1/2 cup of water for 4 to 6 hours or overnight in the fridge.
In a medium bowl, add cashew nuts, melon seeds, fennel seeds, poppy seeds, black peppercorns, cardamom, and edible dried rose petals.
Add about 3/4 cup of water, mix well, cover, and set aside for 4 to 6 hours or overnight.
Peel the almonds and pistachios and add them to a blender.
Transfer the remaining ingredients to a blender along with the water used for soaking. Also, add grated nutmeg and sugar.
Blend to a smooth paste. Add more water if needed to achieve a smooth paste.
Boil milk in a saucepan over medium heat, stirring occasionally.
Add 2 tablespoons of hot milk to crushed saffron in a small bowl. Mix well and set aside.
Switch off the heat and set aside the milk until it turns lukewarm.
Add the saffron-infused milk and thandai masala paste to the lukewarm milk, mix well.
Taste and adjust with more sugar if needed.
Once the milk has cooled down completely, add 1 tablespoon of rose water (optional), mix well. Taste and adjust with another tablespoon of rose water if needed. Cover and refrigerate for a couple of hours or until chilled.
When you are ready to serve strain the thandai using a muslin cloth (the traditional way) or a fine-mesh sieve over a pitcher, pressing back against the solids to extract maximum flavor.
Add some ice cubes or crushed ice to a serving glass, pour the thandai. Garnish with slivered nuts, dried rose petals, and saffron strands. Enjoy!