I've made something like this before but I don't have any set recipe for it, I think I just made it up on the fly as another experimental dish!
I would approach it in the same way as any other BIR style dish, but you'll probably have to experiment with it until you get it right to your taste and/or what you or your friend can remember it tasting like in a restaurant.
I would cook the lentils as you've described but I'd probably also part precook the potatoes in turmeric and a little salt too.
Then you can assemble the dish as you would any other BIR style dish. Heat the oil, pinch of asafoetida, fry for 10 seconds or so, tsp of g/g paste, fry till golden, fry spices, probably mix powder, chilli powder, salt, pinch or so of Garam, ladle of base reduced, perhaps another ladle of base, pinch of fenugreek leaves, perhaps a tsp of tamarind concentrate if you have any, in go potatoes, reduce mixture to coat, perhaps some chopped green chilli's and mix in your lentils. Simmer till potatoes are done, add chopped coriander.
You could make it more complex by adding a tempering to your lentils before you add them. In a tablespoon of oil, fry some coriander seeds, cumin seeds, fenugreek seeds, couple of whole red chilli's and when fragrant, set aside to cool. Empty into a pestle and mortar and grind to a paste, add and stir this into your lentils before adding to your main dish.
Experiment and have fun!