The thing with curries is that the spices need to work. They work through the way they interact with the oil and other ingredients, which all depends on how you cook them to start with.
First thing to remember is that the majority of curry sauces as we know them are based on onions. People make the mistake of thinking that onions are simply an add-on, or padding, so they tend to be sweated in a bit of oil and then forgotten about, but in a curry the onions are treated very much as a spice, and need to be cooked properly in order to develop their part of the mixture.
I do basic curry base which is more or less the same no matter what I’m going to add in later, with only one or two deviations, as taught to me by a colleague and various friends.
Firstly, don’t be afraid of the amount of oil. You need plenty to make it work, as it affects the way the base cooks, and carries the dry spices through the dish. The oil is flavoured at the start with some dry whole spices. I use a large heavy based casserole dish like a le creuset, as it’s great for retaining heat consistently.
I add enough oil to give a good covering, perhaps up to a quarter inch deep, put it on the heat and add a couple of bay leaves, four or five whole cloves, an inch of cinnamon stick, a couple of teaspoons whole jeera (cumin seeds) and three or four cardomum pods. As the oil heats these will eventually start to spatter, and then it’s time to put in the onion. I use a couple of medium onions sliced very thinly and then chopped fine. Stirred into the oil these need to be cooked at a reasonably high heat for a good ten minutes or so, stirred to stop them sticking. They should take on a yellow gold colour after ten minutes, then add plenty of chopped or minced garlic and some ginger. Stir this in and reduce the heat to medium. You want them cooked but not browned or burnt.
At this stage the onions will begin to break down, still looking like chopped onions, but as the garlic and ginger combine with them they
start to look more like a thick paste. After a few more minutes cooking the mixture will split. The paste will form holes, and the oil will bubble up through it. Now it’s time to add a tablespoon of tomato concentrate and stir this into the paste, then half a teaspoon of haldi (turmeric) and a teaspoon of salt. Stirred briskly, now add half a tin of tomatoes and stir these in. I often blitz or sieve them first, but it isn’t vital. Here’s where you will see the onions breaking down completely and giving up their flavour into the mixture. The liquid from the tomatoes needs cooking down, so stir and leave it to cook, again until the mixture splits. You can add in chopped green chillis at this stage if the curry needs them.
From here the basic spice paste is done. It has the flavours from the whole spices and the wet spices combined, and the next stage is where you add the powdered or ground dry spices, all depending on the finished curry.
For a basic chicken massala I would now add red chilli powder or flakes, ground coriander, a touch more jeera and stir it in. This forms a thick, sticky paste and keeping the heat high I’d add in diced chicken, or even better a jointed chicken, bones and all. Stir this so that each piece is coated with the oily spice paste, making sure nothing gets stuck and burns on the bottom of the pan.
Once the chicken starts to colour add half a cup of water. As soon as you do the paste will reconstitute into a thick sauce. Stir then reduce to a simmer and cover the pan. Simmer for as long as it takes to cook the chicken through and watch the sauce. You can add water a little at a time, but it needs to be pretty thick.
It’s done when once again the oil separates and floats to the top. Add chopped green chilli to taste, a handful of chopped fresh coriander and a sprinkle of garam massala.
The long-winded cooking process is designed to extract and develop the flavour of the wet spices, otherwise the onion, ginger and garlic remain harsh and occasionally bitter. I expect a resting period or freezing would help to break down the fibres, sort of replicating the multi-stage process I’ve described.