This recipe calls for frying onion until softened, then pureeing.

I have not posted for a while but this recipe is very very close as a BIR curry sauce.
It just needs time to darken: -

Ingredients
1/2 Cup veg oil
3 Large Onions - Finely Chopped
1 Can Tinned Tomato 400g tin chopped
15 Garlic Bulbs Cloves (yes 15)
1 Cube Ginger
1 Teaspoon Salt
1 Tablespoon - Coriander
1 Tablespoon - Cumin
1 Tablespoon - Chili Powder
1 Tablespoon - Paprika
1 Tablespoon - Turmeric

Method
1. Take a stock pot
2. Heat veg oil
3. Add Onions fry until softened or until they turn a little darker (15 to 20 mins), this should take the acid out of them.
4. Blend onions with 1 tin toms, garlic bulbs, ginger & some water until become a puree / has a soup like texture.
5  Put the puree into the stock pot on a medium heat add 1 teaspoon salt.
6. Cook gently (medium high heat) until starting to become darker, this could be an hour or so, keep it going the longer the better, add water or cup chicken stock iif becomes to thick.
7. Add the ground spices (coriander/cumin/chili/paprika/turmeric) into the puree and stir
8. Cook until the oil rises to the top this is the sign the base sauce is ready, but make sure it is dark.

This is an adaption from this recipe which is real and from a restaurant in Bradford.
I was so impressed with the sauce i have adapted it above and now think this could be the real thing
http://www.curry-recipes.co.uk/curry/index.php?topic=1846.msg16005#msg16005

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