Heat 1/4 cup milk in a small saucepan just until warm and steaming. Remove from heat. Grind the 20 saffron threads between your thumb and finger into the milk. Cover and let steep until you are ready to use it.
Heat 1/4 cup ghee or oil in a large Dutch oven over medium-high heat. Add 2 sliced onions and season generously with 1 teaspoon salt. Let cook, stirring frequently, until the onions are dark golden brown and caramelized. Lower the heat if they start to get too dark too fast, caramelizing the onions should take about 15 minutes. Remove half of the onions to a medium bowl.
Add 1/2 cup cashews and 1/3 cup raisins to the bowl with the caramelized onions. You will use this to garnish the top of the rice. Set aside for now.
To the pot with the remaining caramelized onions, add the 2 teaspoons ginger, 5 cloves minced garlic, 1 minced jalapeño, 2 teaspoons garam masala, 1 cinnamon stick, 1 1/2 teaspoons coriander, and 1 teaspoon turmeric. Add another tablespoon of ghee or oil if the pot looks dry. Fry, stirring frequently, over medium heat for 1 minute.
Add the chopped sweet potato, chopped head cauliflower, water, and 1 teaspoon kosher salt to the pot. Cover and cook until the vegetables are tender enough to be easily pierced with a knife, about 10 minutes. Once they are tender add the 1 cup peas and can of drained chickpeas and cook another 3 minutes. Stir in 1 cup chopped cilantro and mint. Transfer mixture to a large bowl. Taste and add more salt if needed. Clean the Dutch oven.
Par-cook the rice. Combine 1 1/2 cups rice, remaining 2 teaspoons salt, 2 teaspoons cumin seeds, 2 star anise pods, 4 cardamom pods, and 1/2 teaspoon cayenne in a medium saucepan. Fill with water and bring to a boil. Cook 3 minutes then drain rice and spices through a fine mesh sieve. Rice will be partially cooked.
Grease the cleaned Dutch oven. Spread half of the rice on the bottom of the greased dish. Top with vegetable mixture and any liquid in the bowl and spread remaining rice over the top. Sprinkle with the saffron milk and top with the onion-cashew garnish.
Cover pot tightly with foil and then the lid. The foil will keep any steam from escaping. Place over medium-low heat and cook until rice is tender and liquid is absorbed, about 15 minutes.
Remove from heat and let sit 5 minutes. Remove lid and foil and fluff rice with a fork. Serve directly from the pot.