The best eating in Santiago is to be had along the pleasant Rúa de Raiña (off the right side of the Cathedral).
Start with Trafalgar (on the corner of Rúa de Raiña). If you order a draft beer (1.50€), they'll let you try one of their famous Tigres Rabiosos (mussels in hot sauce) for free. If you want a full order, it costs 4.80€.
Then move on to Viñoteca ...