In a bowl, wash the rice gently in several changes of cold water until the water runs clear. Drain it and soak it in warm water for 20 to 30 minutes. Boil 6 cups of water in a large saucepan. Add 1 teaspoon of the salt when the water comes to a full boil. Add the rice and stir a few times, cooking it for 4 to 5 minutes, until it is two-thirds cooked. (The longer the rice has soaked, the less time it will take to cook.) Drain the rice in a sieve, run cold water through it for a minute, and then spread it on a large platter or cookie sheet to cool. Set aside.
Heat the milk over low heat in a small pan until warm to the touch; remove from the heat and add the saffron threads to bloom. Set aside.
Slice 1 onion and coarsely chop the other. In a skillet, heat 3 tablespoons of the ghee over high heat and fry the sliced onion, stirring constantly, until dark brown and crisp, 8 to 10 minutes. Leaving as much of the ghee as possible in the skillet, transfer the browned onions with a slotted spoon to a paper towel-lined plate and set aside.
Add the chopped onion to the same skillet and sauté over medium-high heat until it is lightly browned, 6 to 8 minutes. Transfer these browned chopped onions, along with the ghee from the skillet, to the bowl of a food processor. Add the ginger, garlic, and tomato and process into a paste. Add the yogurt, along with the garam masala, coriander, red chile, turmeric, and 1 teaspoon of the salt and process to blend for 1 minute. Transfer the marinade to a large mixing bowl and add the Cornish hen pieces. Sprinkle with the remaining 1 teaspoon salt, mix well to coat, and set aside for 30 minutes.
Heat the slow cooker on high for 15 minutes before use. Spread the remaining tablespoon of ghee on the bottom and one inch up the sides of the slow-cooker insert. Layer the marinated hen pieces evenly, and use a wet paper towel to clean any marinade off the sides of the insert before continuing.
Sprinkle half the fried onion slices over the hens, then half the saffron milk, along with some of the strands, half the cashews and raisins, and half the sliced serrano chiles. Cover these ingredients with all of the rice and repeat with the remaining fried onions, saffron, cashews, raisins, and chiles.
Cover the insert of the slow cooker with aluminum foil, and fold the foil over the sides of the slow cooker. Place the lid on the cooker and cook on low for 4 hours.
Remove the insert from the cooker and let the biryani stand for about 5 minutes. Serve hot.