Dishoom shared 4 of its recipes here, including the house black dhal - I think it’s great, as you can tweak the recipes when you cook it yourself: reduce the chili or spice content, for example.
The black dhal recipe here called for about 4 hours of slow-cooking, and lots of butter and cream.
When I was in Delhi, India, many years ago, we ate at ITC Maurya Hotel’s famous Bukhara restaurant, which had always claimed that their black dhal was slow-cooked for 15 hours (!) to obtain its rich flavours. Frankly, I think it tasted no different from Dishoom’s - shovelfuls of butter and cream did the trick, rather than the extra-long slow-cooking of the beans. 
