I make an absolutely delicious lamb rogan josh that is better than in a curry house, but still has that curry house flavour IYKWIM (because I think others are right - "real" curry and curry house curry are quite different really. It's a little bit of a cheat, but works well.
Use Lamb shoulder if possible, diced, as it works extremely well if left to cook for hours.
Fry one large onion, sliced, in a small amount of oil (agree with earlier comments about oil quantities). When it is softened, add approximately 500g diced meat and brown then add approx two tablespoons of Patak's Rogan Josh paste and mix in. Add two tins of chopped tomatoes and then beef up the flavourings already present with the paste by adding:
1 chopped red chilli
2 cardomom pods
1 stick cinnamon
a couple of cloves, crushed lightly.
If you want to, you can also add about half a cup of water.
Leave to simmer very very gently for anything from 1 - 4 hours, stirring occasionally and if necessary adding a little extra water. Just before serving, add a bunch of chopped coriander.
You can play with these quantities according to what you're doing - more onion will make it richer and it will go further. More tomotoe might require more cooking time but will make it cheaper etc etc.