Slice the onions so that the are paper thin. You want to almost fill the pan you are using with the onions.
Slice the red bell peppers and place them on top of the onions to fill the pan.
Add the fennel powder and cloves and cover.
Now add the base curry sauce so that it covers the onions and about 200ml water.
Place on a low heat and bring very slowly to a simmer.
This is when I place the onion sauce in the oven on the a very low temperature with the lid on. You can leave it there all night if you like. The longer you cook the sauce the better up to about ten hours.
The top of the sauce will be quite blackened when you take it out of the oven. Try it. If you like it, stir it in. If not, remove the blackened bits.
Place the finished sauce in a blender and blend until smooth or leave it chunky as it is.
Use immediately or freeze.