No there isn't.

You can just order a-la-carte as per the current menu at the restaurant. Order 2 main courses from the menu, and one of them will be for free. Remember, you can't order a starter and a main course and expect the main course to be complimentary.  

Certain restaurants only offer set menus, if this is the case, the specific privilege is highlighted on the restaurant page in your book. E.g. this can be 20% off the total food bill as you can see on the image below. 

If there's no privilege listed on the restaurant page that means the standard privilege applies: the second main course is for free.