Place onion, garlic, ginger and chiles in a food processor and chop to form a paste. Heat oil in a Dutch oven and add onion mixture and the cassia leaves. Fry over high heat for 5 minutes, reduce heat to medium and fry another 2 minutes. Onions should be just starting to brown. Add the lamb and stir until all pieces are coated with onion mixture. Fry 15 minutes, stirring constantly.