Heat oil and saute beef and onions for 15 minutes. Combine tamarind paste with beef stock and tomato paste. Allow to stand for 10 minutes. Add mixture to beef along with paprika, coriander, fenugreek, tarragon, salt and pepper. Reduce heat to low and simmer, covered until beef is very tender about 1 1/2 hours. Add more stock if needed. Stir in garlic, walnuts and cilantro. Simmer 5 minutes. Add sugar, simmer for 10 minutes.