Cut chicken thighs into 1.5-inch pieces. In a large bowl, combine chicken with yogurt, ginger-garlic paste, green chilies, half of the mint and cilantro, 1 tsp salt, turmeric, red chili powder, biryani masala, and garam masala. Mix well to coat every piece. Cover and refrigerate to marinate for at least 30 minutes, or up to 4 hours for deeper flavor. While marinating, thinly slice the onions.

Heat ghee or oil in a large heavy-bottomed pot or Dutch oven over medium-high heat. Add the thinly sliced onions and spread them evenly. Fry, stirring occasionally, until they turn deep golden brown and crispy. This takes patience but is crucial for biryani's signature flavor. Remove about two-thirds of the fried onions with a slotted spoon and set aside for layering and garnish.

Rinse basmati rice thoroughly under cold running water until the water runs clear (about 3-4 rinses). In a separate large pot, bring plenty of water to a rolling boil with 1 tsp salt and the whole spices (cardamom, cloves, cinnamon, bay leaf). Add the rinsed rice and cook for only 5-7 minutes until it's about 70% cooked (the grains will be slightly soft but still have a bite). Drain immediately in a colander.

In the same pot used for frying onions (with the remaining fried onions and oil), add the marinated chicken. Stir well and cook over medium-high heat for 10-12 minutes, stirring occasionally, until the chicken is partially cooked and the yogurt sauce thickens. This forms the flavorful base for your biryani.

Preheat your oven to 350°F (175°C) if using the oven method, or prepare your stovetop for low heat. Sprinkle half of the reserved fried onions, remaining fresh mint and cilantro over the chicken base in the pot. Then, gently layer the par-boiled rice evenly on top of the chicken. Dissolve saffron strands in warm milk and drizzle over the rice. Add a tablespoon of lemon juice for brightness.

Cover the pot tightly with its lid. For a perfect seal, you can seal the edges with aluminum foil or a strip of dough before placing the lid. Place the pot in the preheated oven for 25-30 minutes, or cook on the stovetop over very low heat (using a griddle or tawa underneath for even heat) for 30 minutes. This 'dum' cooking allows the rice to fully cook in the aromatic steam.

Once the 'dum' cooking is complete, remove the pot from the heat or oven. Let it rest, still covered, for 10-15 minutes. This allows the flavors to meld and the rice grains to firm up. Gently open the lid and fluff the biryani using a fork, mixing the chicken from the bottom with the rice. Garnish with the remaining fried onions, fresh mint, and cilantro. Serve hot with raita (yogurt dip) or a simple salad.