Q:

What is the best way to cook the beef cut "nerveux gite"? 

In France, just bought a beef cut of meat named "nerveux gite a bourguignon", planning to cook it in the slow cooker.   However, it seems incredibly tough!   What's the best way to prepare it? 

A:

Welcome!  Braising would be the preferred cooking method for that cut of beef.  The long, slow cook will result in falling apart, tender, moist meat.  
Because of the name of the cut, the first thing that comes to mind is beef bourguignon.  From  Wikipedia :

It is a stew prepared with beef braised in red wine, traditionally red Burgundy, and beef broth, generally flavoured with garlic, onions and a bouquet garni, with pearl onions and mushrooms added towards the end of cooking. 

If you decide to make beef bourguignon you can do a search to decide on a recipe.  Julia Child's version is quite famous.  
However, the bottom line is that you want to braise the beef.  So you could use it in any recipe where braising is the cooking method called for.