Combine almonds, seeds, cashews and pistachios with 2 cups of water and let soak for one hour.
Remove and discard skins from almonds and pistachios. Drain nuts and seeds and set aside.
Bring milk to a boil in a 2-qt. saucepan. Add sugar and saffron, stirring until sugar is dissolved; set aside.
Toast the fennel, peppercorns, cardamom and cinnamon in an 8″ skillet over medium heat until fragrant, about 4 minutes; let cool slightly. Transfer spices to a food processor, along with the nut mixture, 1 tbsp. milk mixture, and rose petals. Puree to a smooth paste.
Whisk paste into milk mixture, and strain through a fine mesh sieve, pressing on solids.
Refrigerate strained milk mixture until chilled. Stir in gin and divide between serving glasses; garnish with grated nutmeg and rose petals, if you like.