1. Pour 1 tablespoon oil into a 10- to 12-inch pan over medium heat. Add cashews, cardamom, bay leaves, cinnamon stick, and cumin seeds; stir until fragrant, about 2 minutes. Pour into a bowl.
2. In pan, heat remaining tablespoon oil over medium-high heat. Add onions and garlic; stir until limp, 5 to 8 minutes.
3. Stir in shrimp, chili powder, turmeric, pepper, 1/2 teaspoon salt, and nut-spice mixture. Cook, stirring frequently, just until shrimp begin to turn pink, 2 to 3 minutes. Reduce heat to medium; stir in yogurt. Cook, stirring often, until most liquid has evaporated, about 15 minutes. Remove bay leaves and cinnamon stick.
4. Meanwhile, in a 5- to 6-quart pan over high heat, bring 2 1/2 quarts water to a boil. Add rice; boil uncovered until almost tender, 7 to 8 minutes. Drain and pour into a bowl. Stir in the lemon juice and salt to taste.
5. In a buttered 3 1/2-quart baking dish, spread about 1/3 of rice mixture level. Top with 1/2 shrimp mixture, spreading level. Repeat layers, ending with rice. Sprinkle evenly with saffron threads. Cover tightly with foil.
6. Bake biryani in a 350° oven until center is hot and rice is tender to bite, 20 to 25 minutes. Serve immediately, with raita.