Prepare the Nuts: Using the flat side of a large knife or a small food processor, coarsely chop the pistachios or pumpkin seeds.
Set aside.
Infuse the Yogurt: In a medium bowl, vigorously stir the yogurt, milk, and chopped mint together for 1-2 minutes until fragrant.
Season with a pinch of salt. Push the mixture through a fine-mesh strainer to remove the mint leaves.
Cover and refrigerate the infused yogurt.
Marinate the Melon: In a separate bowl, combine the grated melon, citrus juice, and chili powder.
Season with salt and pepper.
Stir to combine, then cover and refrigerate for at least 2 hours, stirring occasionally. The melon will release its juices, which will form the base of the soup.
Assemble and Serve: When ready to serve, taste both the yogurt and melon mixtures and adjust seasonings if needed. Spoon the marinated melon and its juices into chilled bowls.
Top generously with the mint-infused yogurt and finish with a sprinkle of the chopped nuts and a mint sprig for garnish.