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 blender.
Melt the ghee in a large skillet over medium heat; fry the onion in the melted ghee until golden brown, about 5 minutes. Add the red chile pepper paste and lamb to the skillet; 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 skillet, 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 blender. Hold down the lid of the blender with a folded kitchen towel and carefully start the blender, 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 skillet. 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 skillet, place over medium heat, and cook until the soup is thoroughly reheated, about 5 minutes. Serve in warmed bowls; garnish with lemon slice.