Soak 50 grams of almonds, 30 grams of cashews, 20 grams of pistachios, 20 grams of melon seeds, and 2 tablespoons of poppy seeds in water for 4-6 hours or overnight.
Drain the water and grind the soaked nuts and seeds into a fine paste using a little water or milk.
In a large bowl, mix 1 liter of chilled milk, the ground paste, 1/2 cup of sugar, 1/2 teaspoon of cardamom powder, 1/4 teaspoon of saffron strands, and 1/4 teaspoon of black pepper powder. Stir well until the sugar dissolves.
Strain the mixture through a fine sieve or muslin cloth to remove any lumps or coarse particles.
Pour the strained thandai into glasses and garnish with chopped almonds, pistachios, and a few saffron strands.