Clean and wash the urad dal thoroughly, then place it in a bowl and cover with 2 inches (5 cm) of water to soak for 4–6 hours before rinsing and draining again.
Transfer the drained dal to a food processor with 0. 67 cups (160 ml) of water and process for 4–5 minutes until the purée is smooth and fluffy, adding a splash more water only if necessary to keep the blades moving.
Incorporate the asafetida, black pepper, and 0. 5 tsp (2 ml) of salt into the dal purée, mixing well and dividing the paste into equal portions based on the number of leaves you have prepared.
Blanch the trimmed leaves in boiling water with a pinch of baking soda for 4–5 minutes to remove bitterness, then rinse in cold water and set aside.
To assemble the packets, lay a leaf (or half a large leaf) lighter-side down and spread an even layer of dal purée (0. 125–0. 25 inch / 3–6 mm thick) over the surface, leaving a 0. 5-inch (1. 5 cm) border.
Fold the leaf in half lengthwise, spread another layer of purée on top, and fold again into quarters to form a small, square packet, patting gently to ensure the dal is evenly distributed.
Steam the prepared packets in a steamer for 15 minutes, then allow them to cool (you can refrigerate them for up to 8 hours at this stage if making ahead).
Heat the ghee or oil in a large frying pan over moderately high heat and fry the packets in a single layer for 5–6 minutes per side, pressing down with a slotted spoon until they are richly browned and crisp.
Remove the fried packets to a strainer to drain, then cut each one into 1. 5-inch (4 cm) squares and arrange them in a single layer in a buttered baking dish.
Use the remaining oil in the pan to fry the cumin and mustard seeds until the mustard seeds pop, then add the tomato pulp, sliced tomatoes, turmeric, coriander, garam masala, paprika, and salt.
Simmer the tomato sauce for 15 minutes before pouring it over the arranged leaf squares and spooning the yogurt or sour cream across the top.
Bake the dish in a preheated 350°F (180°C) oven for 25 minutes until the top is beautifully browned, then let it rest for 15 minutes to firm up before serving garnished with fresh herbs.