There are many, many excellent expensive French restaurants in Las Vegas, but the OP asked for casual French food, so I’m not going to post them. Besides they’re well documented, a quick search on google or even Yelp can give you a good idea.
Off the top of my head:
Robuchon a La Maison
L’ Atelier de Robuchon
Twist
Restaurant Guy Savoy
and Bardot Brasserie can qualify for both fancy french and french bistro.
There are many others I’m not mentioning, my apologies.
