Cut meat into long strips and then even-sized pieces of about 3cm. Place in a large bowl.
Grind onions, garlic, galangal, ginger, chillies, lemon grass and turmeric to a fine paste. Add paste to the meat. Add coriander and cumin seeds. Mix well. Add lime leaves. Cover and leave in a cool place to marinate. Strain tamarind and reserve the juice.
Place spiced meat and soy sauce into wok. Stir in coconut milk, water and tamarind juice. Season to taste with salt. Stir until liquid comes to boil, then reduce heat to simmer gently, half-covered, for 1.5 to 2 hours or until the meat is tender and the liquid is reduced. Add potatoes 20-25 minutes before end of cooking time.
Slice as finely as possible. Spread out in a single layer on kitchen paper. Leave to dry in an airy place for 30 minutes to 2 hours.
Heat the oil in a wok to 190 degrees. Fry the onions in batches until crisp and golden, turning all the time.
Drain on kitchen paper and allow to cool.
Transfer beef rendang to bowl and sprinkle with deep fried onions.