pH, have you tried Kris Dhillon's Saag Paneer? That sauce is a good starting point. (She also has a recipe for Saag Meat, but that sauce doesn't sound as good.) I follow the recipe almost exactly, (chilli to taste), keeping in mind that Kris uses rounded spoonsful and 5 oz. teacups. My main change is adding about 1/2 cup cream to the pureed spinach at the end to make it rich and creamy.
BTW, she uses canned spinach, which I normally HATE. But as you say, restaurant saag is dark, like canned spinach. So that's what I use, and it works, what can I tell you?!
-Mary