Wash the chiles in cold water. Remove veins, stems, and seeds. Tear the pods roughly into pieces, place in a bowl and soak them in hot water, at least 1/2 cup, for an hour. Place the soaked chiles and their soaking liquid in a blender jar and blend to a thick puree.
Assemble the stew:
Place the lamb, tomatoes, garlic, raisins, spices, and seasoning to taste in a heavy flameproof casserole which has a lid.
Add enough water to the chile puree to make it the consistency of thin cream *(about 1 1/2 cups). Pour the puree over the meat, cover the casserole, bring to a boil. Reduce the heat and simmer very gently until the lamb is tender when pierced with a fork, about 2 hours.
Five minutes before serving, add the sherry. Sprinkle with almonds just before serving.