Grind the red chillies, coriander seeds, cumin seeds, turmeric powder, nutmeg, cloves, peppercorns, curry leaves, garlic and ginger into a smooth paste using a food processor or liquidiser.
Melt the ghee in a large frying pan over medium heat; fry the onion in the melted ghee until golden brown, about 5 minutes. Add the red chilli paste and lamb to the frying pan; cook and stir until the lamb is browned, about 5 minutes. Season with salt. Stir the lamb stock and tomato puree into the lamb mixture; reduce heat to medium-low, cover the frying pan and simmer until the lamb is tender, about 30 minutes. Add the carrot and apple; cook another 15 minutes. Remove from heat and allow to cool slightly.
Remove the lamb from the mixture and set aside. Pour the rest of the soup into a liquidiser. Hold down the lid of the liquidiser with a folded tea towel and carefully start the liquidiser, using a few quick pulses to get the soup moving before leaving it on to puree. Strain the soup through a fine mesh strainer back into the frying pan. You ARE allowed to press on the contents of the strainer! Remove and discard any large and hard pieces. Add the lamb back to the frying pan, place over medium heat and cook until the soup is thoroughly reheated, about 5 minutes. Serve in warmed bowls; garnish with lemon slice.