I've already mentioned the Secret Chinese below Plaza de España, and Despensa de Carmen on Calle Santiago. Both great value.
On a South American/Latino flavour (specifically Ecuador) try El Paso de Jean for a fabulous sit down menu of the day for just €8. Big hearty stews and soups - cream of pumpkin, fish caldo, pigs trotter caldo, creamy cheese vegetable stew. All delicious and garnished with a generous sprinkle of coriander. Followed by a fish, or meat course often served with lentils and rice, or creamed potato. Drink included with a choice of yoghurt, flan, or coffee to finish in a fun, friendly and lively Latino atmosphere.
Perfect now winter has arrived.