These are some of my favorite off strip restaurants:
Any restaurant in Chinatown Plaza (or surrounding area):
Joyful House-not far from Chinatown Plaza( 4601 Spring Mountain Rd) has great authentic Chinese food. They also have a great hot pot from about 10PM until 3AM.
Lotus of Siam:
Someone mentioned Pasta Mia (W Flamingo). In the same strip mall-a few door down- is Thai Spice which also has very nice Thai food:
No list would be incomplete without mentioning Ellis Island that has a very good steak meal ($6.99- off menu). Others have praised the other items on a menu -including breakfast:
My very favorite off strip restaurant is Rosemary's. If you look at the promos you will see some great values-but it is good without the promos.