Wash the Basmati rice thoroughly and soak it in water for about 30 minutes. Drain the rice and set it aside.

In a large pan or pot, heat the ghee or vegetable oil over medium heat. Add the sliced onions and sauté until they turn golden brown. Remove half of the fried onions and set them aside for garnishing.

To the remaining onions in the pan, add the ginger-garlic paste and cook for a minute until fragrant.

Add the chicken or mutton pieces to the pan and sauté until they are browned on all sides.

Stir in the biryani masala powder, turmeric powder, red chili powder, and salt. Mix well to coat the meat with the spices.

Add the yogurt to the pan and cook for a few minutes, allowing the meat to absorb the flavors.

Add enough water to the pan to cook the meat until tender. Cover the pan and simmer until the meat is cooked through. If using chicken, it usually takes around 20-25 minutes, while mutton may take longer.

In a separate pot, bring water to a boil and add the soaked and drained Basmati rice. Cook the rice until it is 70% cooked. Drain the rice and set it aside.

Preheat your oven to 350°F (175°C).

In a large baking dish, layer half of the partially cooked rice at the bottom. Sprinkle some saffron milk, rose water, mint leaves, and coriander leaves over the rice.

Place the cooked meat and potatoes on top of the rice layer. Cover it with the remaining rice.

Garnish the top layer with the reserved fried onions, saffron milk, mint leaves, and coriander leaves.

Cover the baking dish with aluminum foil and bake it in the preheated oven for about 30-35 minutes.

Remove from the oven and let it rest for a few minutes before serving.

Serve the Kolkata Biryani hot with raita (yogurt dip) or a side salad.