In a dry skillet, roast cilantro stems.
In a food processor, combine cilantro stems, 1 of the onions, ginger, garlic, cumin, coriander and fennel.
In a deep skillet, heat mustard oil. Add cinnamon, green cardamom, black cardamom, cloves and peppercorns. Fry until it becomes fragrant.
Add onion and saute until it becomes golden brown.
Add chicken and masala paste from the food processor. Add turmeric, chile powder and tomatoes. Simmer for 15 minutes.
Add sugar and salt. Simmer, covered, for 15 minutes.
Add lemon juice. Stir to combine.