Step 1: Prepare Crispy Onions

Thinly slice the red onions using a mandoline for consistency. Heat about 2cm of vegetable oil in a pan over medium-low heat. Fry the onions in batches, stirring constantly, until they are golden brown. Remove with a slotted spoon and drain on a paper towel-lined tray. Set aside.

Step 2: Marinate the Chicken

In a large, heavy-bottomed pot, combine the chicken thighs, Greek yogurt, Kashmiri chili powder, turmeric, garam masala, ginger paste, garlic paste, juice of one lemon, sliced green chili, a cinnamon stick, 8 lightly crushed green cardamom pods, cumin seeds, cloves, and salt. Add roughly chopped mint and coriander, and about one-third of the crispy fried onions. Mix everything thoroughly to coat the chicken. Cover the pot and let it marinate in the refrigerator for at least 4 hours, or up to 24 hours for best results.

Step 3: Prepare the Rice

Take the aged basmati rice and wash it twice with cold water. On the third wash, cover the rice with water and let it soak for 35-40 minutes.

Step 4: Prepare Saffron Milk and Dough Seal

While the rice soaks, add a generous pinch of saffron threads to a small bowl of warm milk, stir, and set aside to infuse. In a separate bowl, mix all-purpose flour with enough water to form a workable, slightly sticky dough. Knead it briefly and roll it into a long log shape, sufficient to encircle the rim of your cooking pot. Set this dough aside.

Step 5: Par-boil the Rice

Bring a large pot of water to a rolling boil. Season generously with salt and add aromatics like a few green cardamom pods, a halved green chili, a cinnamon stick, 2 cloves, and a teaspoon of cumin seeds. Drain the soaked rice and add it to the boiling water. Cook for 4-5 minutes until the rice is about 70% cooked (al dente). It should still have a slight bite.

Step 6: Start Cooking the Chicken and Layering

Place the pot with the marinated chicken on high heat and cook for 4-5 minutes, stirring to start the cooking process. Turn off the heat. Remove half of the chicken pieces and set them aside. Spread the remaining chicken evenly at the bottom of the pot.

Step 7: Layer the Biryani

Drain the par-boiled rice and add half of it in an even layer over the chicken in the pot. Place the reserved chicken pieces on top of this rice layer. Sprinkle a generous amount of the remaining crispy onions, and some chopped mint and coriander over the chicken. Top with the final layer of rice, spreading it evenly.

Step 8: Final Touches and Sealing

Sprinkle the last of the crispy onions and fresh herbs over the top layer of rice. Drizzle the saffron-infused milk all over. Place the lid on the pot. Use the prepared dough to seal the gap between the lid and the pot, pressing firmly to ensure no steam can escape.

Step 9: Dum Cooking

Place the sealed pot on maximum heat for 10 minutes. After 10 minutes, reduce the heat to the lowest possible setting and continue to cook for another 30 minutes. This slow-cooking process is called 'dum'.

Step 10: Rest and Serve

After the cooking time is complete, remove the pot from the heat and let it rest, still sealed, for 20 minutes. To serve, carefully break the dough seal and remove the lid. Gently fluff the rice from the sides and plate the biryani, ensuring you get portions from all the layers. Garnish with a final sprinkle of fresh coriander and mint.

• Use a mandoline for consistent, thin onion slices which helps them fry evenly.

• Fry onions on a medium-low heat to allow for caramelization without burning.

• Remove fried onions from the oil just before they look perfectly done, as they will continue to cook from the residual heat.

• Using aged basmati rice is crucial for achieving separate, fluffy grains in the final dish.

• Wash the rice thoroughly until the water runs almost clear to remove excess starch.

• Cook the rice until it's only about 70% done (al dente) because it will finish steaming in the pot with the chicken.

• Ensure the pot is sealed very well with dough or a tea towel to trap all the steam, which is essential for the 'dum' cooking process.

• Instead of chicken thighs, you can use chicken drumsticks or a whole chicken broken down into pieces.

• For a vegetarian version, you can omit the meat and use vegetables instead.

• If saffron is unavailable or too expensive, you can use a pinch of turmeric in the warm milk to achieve a similar yellow color for the rice.

• As an alternative to the dough seal, you can place a clean, damp tea towel under the lid to create a tight seal.