Soak almonds, cashews, melon seeds, and poppy seeds in water for 4–5 hours.
Peel the almonds and grind all soaked ingredients into a smooth paste.
Add black pepper, fennel seeds, cardamom, and a few saffron strands while grinding.
In a pan, boil milk and add sugar. Let it cool slightly.
Mix the prepared paste into the milk and stir well.
Strain the mixture to remove any coarse particles.
Chill the Thandai in the refrigerator for a few hours.
Serve cold, garnished with chopped nuts and saffron strands.