The restaurant that I recommend to everyone when you are hungry after going out to the theater, is Plomb du Cantal. Their specialty is aligot, mashed potatoes and cheese served with top notch meats.
This is the place to go when you want to literally pig out—with large helpings.
There is one in Montparnasse right across from the Théâtre Bobino and another near the theaters of the rue Saint-Martin.