I've been to Santiago twice on short trips and ended up eating at the same places (by accident) both times. One is a steak place called Eladio's ( 3 locations and I don't know which one we went to) and one is a seafood restaurant in the market called Donde Augusto. Both very good and recommended by my Chilean family members. If the Hyatt still has a Thai restaurant, that's good too. The central market is fun with a lot of places to get fresh seafood.
Anything with avocado on it is good, and I was especially fond of the Churrasco sandwich- thin sliced steak, cheese, avocado and mayo.