Pakistani Lamb Biryani

1 cup canola oil
3 large yellow onions, thinly sliced
2 tbsp. garam masala
1 tsp. crushed red chile flakes
1⁄2 tsp. turmeric
18 black peppercorns
9 pods green cardamom
3 pods black cardamom
2  2" cinnamon sticks
6 cloves garlic, minced
6 tomatoes, cored and minced
5 serrano chiles, stemmed and minced
1  1 1⁄2" piece ginger, peeled and minced
2 lbs. trimmed lamb shoulder, cut into 2–3" pieces
Kosher salt, to taste
1⁄2 cup plain yogurt
3⁄4 cup roughly chopped mint leaves
1⁄4 cup roughly chopped cilantro
40 threads saffron, crushed (heaping 1⁄2 tsp.)
2 1⁄2 cups white basmati rice, soaked in cold water for 30 minutes, drained
1⁄2 tsp. cumin seeds
4 whole cloves
2 dried bay leaves

 Heat 1⁄4 cup oil in a 5-qt. skillet over high heat. Add onions; cook, stirring occasionally, until dark brown, 20–25 minutes. Transfer to a bowl; set aside.
Heat remaining oil in a 5-qt. pot over high heat. Add garam masala, chile flakes, turmeric, 10 peppercorns, 5 green cardamom pods, 2 black cardamom pods, and 1 cinnamon stick; cook, stirring, until fragrant, about 1 minute. Add garlic, tomatoes, chiles, and ginger; cook, stirring, 2–3 minutes. Add lamb, season with salt, and cook until lightly browned, about 5 minutes. Cover, reduce heat to medium; cook until lamb is tender, about 1 hour. Add fried onions,   yogurt, 1⁄2  cup mint, and 2 tbsp. cilantro; cook, uncovered, for 15 minutes more. Set aside.

 Put saffron into a bowl and cover with 1⁄2 cup hot water; set aside. Bring 4 cups of water to a boil in a 5-qt. saucepan. Add remaining peppercorns, green and black cardamom, and cinnamon, along with the rice, cumin, cloves, and bay leaves, and season with salt. Cook rice until al dente, 5–10 minutes; drain rice and set aside.

 Transfer half the lamb curry to a 5-qt. pot. Top lamb with half the rice. Pour half the saffron mixture onto rice; mix into rice with your fingers. Top with remaining lamb curry and remaining rice; drizzle with remaining saffron; mix. Steam, covered, on low heat until rice is tender, about 10 minutes. Garnish with remaining mint and cilantro.