Add almonds, cashew nuts, whole black peppercorns, raisins, whole green cardamoms, poppy seeds, fennel seeds, melon seeds, and saffron to a bowl.
Add water to the bowl and keep it over the counter for 5-6 hours or overnight.
Add the soaked ingredients to a blender along with the leftover water and blend to make a fine paste.
Add 2-3 tablespoon of milk if required while blending.
Heat milk in a heavy bottom pan over medium heat. Stir frequently to prevent scorching.
When the milk comes to a boil, stir in the paste that we made earlier, granulated sugar, and rose water.
Cook for 2-3 minutes. Stir frequently. Check for sugar and add more if required and cook for another minute.
Remove the pan from heat and let the mixture cool down for 10 minutes. Refrigerate the milk mixture for 3-4 hours. Pass the mixture through a fine-mesh strainer and discard the leftover pulp.
Pour the Thandai into serving glasses. Garnish with rose petals and slivered dried nuts. Serve chilled.