Donde Augusto is consistently recommended as the best of the seafood restaurants inside the Mercado Central's main hall. It is not a fine-dining establishment — the decor is market tables and white tablecloths, the service is quick and efficient — but the seafood is as fresh as it gets in Santiago and the cooks know what they are doing with it. Order the paila marina (a mixed seafood broth of extraordinary richness), the congrio frito, or whatever the fishmongers are recommending that day.
The key is to sit in the central hall at Donde Augusto rather than at the tourist-facing restaurants around the perimeter, which serve average food at inflated prices to people who don't know the difference. Lunch only; go before 1 p.m. on weekdays to get a table without a long wait.