Preparing the purée: cut half the onion, chilies and ginger into cubes. Grind until it turns into purée. Put aside.
Place tamarind into a bowl and add warm water into a bowl. Squeeze the juice. Set aside.
Slice rest of the onion. Set aside.
Heat oil in a pot. Cook the chili purée for 15mins, stirring continuously. Followed by adding the sliced onion. Cook for another 20mins on med heat. Add tamarin juice, salt & sugar to taste.
Place in a cleaned bowl. Serve with beef rendang coconut rice. Note: the core taste should be sour and somewhat sweet.