Combine the chicken, yogurt, ginger-garlic paste, biryani spice, salt, and sliced onion in the slow cooker.
Cover and cook on LOW for 4 to 5 hours, until the chicken reaches 165°F.
During the final 30 minutes, rinse the basmati rice until the water runs mostly clear. In a saucepan, bring the 2 cups broth and 1 cup water to a boil, stir in the rice, cover, and simmer for 15 minutes.
Remove the rice from the heat and let it steam, covered, for 10 minutes. If still firm, add 1 to 2 tablespoons hot water and cook briefly until tender.
Fluff the rice, fold it gently into the cooked chicken mixture, garnish with cilantro, and serve.