After some experimenting recently i believe to release the full sweetness of the onions they need to be boiled in Veg Oil for around 40 Min's at a high heat, this will virtually caramelise them bringing out the natural sweetness and depth we are looking for in our base.
I am convinced if you do this first then add the other ingredients into your base sauce. Water or stock, Tomatoes, pepper garlic, ginger and spice mix this might just give you the base your looking for.
I Will check this out at some stage an report back.
I am thinking 4 large onion in 750ml of oil should do the trick.
Stew