Saw this in Atul's cookbook and he says they go through liters of the stuff in there curries.
Basically base gravy.
3 tablespoons oil
5 black peppercorns
2.5cm piece of cinnamon stick
2 green cardamom pods, bruised
1 bay leaf
1 teaspoon cumin seeds
2 cloves
500g onions, finely chopped
1 teaspoon G&G paste
1 teaspoon ground coriander
1/2 teaspoon ground cumin
1/2 teaspoon garam masala
1/2 teaspoon turmeric
350ml water
1 tablespoon tomato puree, mixed with 4 tablespoons water
sea salt
*G&G paste is ginger and garlic blitz with 10% of the total weight in water*
Heat the oil in a saucepan, add the whole spices and saute over a medium heat until they crackle. Add the onions and saut for 5-8 minutes until lightly coloured.
Add the ginger garlic paste and saute over a low heat for 2-3 minutes, or until the raw smell of the paste disappears.
Stir in the ground spices and saute for a further 2-3 minutes.
Remove and discard the cinnamon stick. Transfer the mixture to a blender or food processor, add 225ml of the water and blitz.
Add the diluted tomato puree, the remaining water and salt to taste, and blitz again until blended.
Transfer to a saucepan and simmer over a low heat for 15-20 minutes, stirring occasionally.
Add more water if the mixture starts to catch on the bottom of the pan, but the idea is to gently fry the sauce, which will darken in colour to light brown. The final texture (not colour) should be something like tomato ketchup. While cooking, it will gloop occasionally and splatter. It can burn or scald, so cook the sauce with caution. Set aside to cool completely, then store in an airtight container in the fridge for up to 3 days. Do not freeze.