Over a decade since this question was asked, let me throw in my own take on this.
Note: This is not a BIR recipe. It's a definite home version, one pot not four. They use bulk methods that you simply cannot reproduce at domestic quantities.
It is also a blend of traditional 'Indian' & 'modern BIR'.
Initially, Rogan Josh was two distinct recipes, made by two distinct populations, split by country of origin as much as by religion, Mughals descended from Persian immigrants & natives of Hindustan. The modern BIR is already a blend of these origins, further anglicised by the use of tomato purée, from a time when you couldn't just walk into a British supermarket to buy Kashmiri mirch, or indeed most of the spices we can now obtain easily, separately, whole or ground. They would use chilli powders [instant Madras heat at the end], curry powders & garam masalas, pre-blended, much more readily available on import, and more easily controllable in large, repeat quantities.
One more thing: If you're going to use supermarket spices, at least use new jars. They're already a bit weak compared to a good importer, but if you keep them in the back of the cupboard for a year or more, they'll be like cardboard.
I was going to laboriously write down the full ingredients list & method, but as there are so many recipes available online now, I've taken this one from BBC Good Food as my start-point & modified it quite a bit, so it's no longer just a verbatim copy/paste…
For the rogan josh paste
1 bunch fresh coriander, stalks and leaves separated
1/2 tsp cayenne powder, or to taste [you can add more of this right up to the last few minutes if you need.]
2 tsp coriander seeds, toasted and ground
2 tsp cumin seeds, toasted and ground
1 tsp fennel seeds, toasted and ground
1 tsp black peppercorns, roughly ground
2 tsp paprika, or preferably Kashmiri mirch, now more readily available
1 tsp turmeric
2 tbsp tomato purée
1 tsp salt, or to taste
For the lamb
2-3 tbsp groundnut [flavourless] oil or preferably ghee [BIR uses far more oil/ghee (& salt) than you would normally feel comfortable with at home.]
5cm/2in cinnamon stick
3 dried bay leaves
5 cardamom pods, lightly crushed
[personally, I use powdered [home-ground] cardamom & cloves so you don't have to pick the bits out later, though I add half at the start & the rest an hour before serving, so the flavours don't vanish]
2 large onions, 1/2 roughly chopped, the rest as fine as you can get it
600g/1lb 5oz lamb neck fillet, all visible fat removed, cut into 3cm/1in chunks
2 garlic cloves, peeled and crushed
1 thumb-sized piece fresh root ginger, peeled and finely grated
[A BIR would use frozen ginger/garlic paste in cubes, or garlic powder.
100g/3½oz fat-free natural yoghurt [optional]
2 tsp garam masala [This, like the clove/cardamom you can split half & half - half now, half an hour before serving.]
1 fresh tomato cut into wedges [optional, but very BIR these days.]
To make the paste, in a mixer, blend together the coriander stalks, red chilli, ground spices and salt.
Mix in the tomato purée.
Heat the oil in a large heavy-based casserole or heavy-lidded saucepan.
Fry the cinnamon stick, cardamom pods, bay leaves, fennel and cloves for 1 minute.
Add the spice paste and fry gently for another 2 minutes or so, stirring, until the oil starts to separate.
Add the onion and lamb and fry for 5-10 minutes or more, stirring & turning up the heat as it will let a lot of water out at this point.
Add the garlic and ginger and continue cooking until you can no longer smell raw garlic.
Cover with a lid and simmer as low as possible for 2 - 4 hours, until the lamb can be easily broken down with just a fork.
For the first hour keep checking your liquid level. The sauce will at first thin as more water comes from the onion & meat, but will eventually reduce again. You don't want it to dry out & consequently burn, but you don't want it swimming. Add a little water if needed.
Your sauce at the end is going to be predominantly onion & ghee/oil, not added water.
Add garam masala an hour before serving, if you add it all at the start the aromatics will have boiled off by the end.
In the last 10 minutes, stir in the yoghurt & tomato wedges.
The fresh coriander leaves you can either gently fold in in right at the end or use as garnish.
In this, I haven't really concentrated so much on quantities as method. Starting with your dry seeds first you do risk burning, but if you do that right at the start it's not too terrible to start over. Adding the wet paste second then keeps your burn risk down a long way. Long cook with lots of onion & oil is what makes your sauce & your full flavour. Water doesn't. Your actual spice blend you can modify over time; that's actually the easy bit, once you get used to it.
Oh, one more thing… You wouldn't always get these in any given dish, but almost all BIR curry houses smell of fenugreek & asafoetida [hing], so you will know them by association.
You can add half to a teaspoon of fenugreek powder any time after your onions go in, for instant 'oooh, it's just like a curry' aroma.
Asafoetida 1/4 to 1/2 tsp ground [it comes in different 'strengths' because it's bulked to make it easier to use]. Be warned. It stinks like something rotten when you first expose your nose to it, but it cooks in a bit like onion flavour. Don't let the initial smell put you off.
If you got this far, you might also like my answer on Why does my curry taste flat which sets out some basics, ingredient types & methodology.