I watched food safari (of course ) and the "sauce" like you see is self generated from the long cooking time as it is indeed cooked from scratch.
In your OP you said you were interested in getting something like the restaurant curries, and they add a gravy.
However, indeed if you have a slow cooker / crockpot you can also do a curry in that with a lot of fried onions, meat etc - all those ingredients I listed in my post above can be put in with the meat and spices and let it simmer for six hours whatever. In the end it will also turn out nice and thick, but a different taste to the pan cooked version, less caramelised but quite smooth and rich.
The trick is not to add water as such, always use coconut cream, tomato passata, ground almonds, onions - allow all the liquid to come from the ingredients. Also as you cook it the meat or chicken will exude juice that blends in as well but it won't end up thin and watery.
Tell you what, I'm doing a butter chicken for din dins, starting right now so I'll take a few shots and post here for the "gravy" method.