One night in Madrid - Where to eat?
I live in Madrid and as you are only there for one night, I recommend you hop in a taxi in front of El Prado, and tell them to take you to Calle Maldonado, esquina con Calle Castelló (Maldonado, corner with Castelló). It´s not far, and the taxi will cost about 5 euros.
The restaurant is called La Cazorla - book, because it´s very popular for it´s delicious and reasonably priced Andalucian-style food. The phone number is 915627288. You will be the ONLY foreigners there - the Spanish dress smartly when they eat out; be warned!
I recomment the Ensalada Cazorla, and the Fritura mixta de la Casa (mixed fried fish). The portions are HUGE - two people would struggle to finish those two dishes if they shared them - both dishes come to about 20 euros. Also good is... everything! Expect to be charged for the bread and aperitíf (2.50 each diner) - it´s a small cover charge - you only need leave a small tip in Spanish restaurants.
I always take visitors to Madrid here, and they all LOVE it - good luck whereever you go xx
If you go up the Calle del Prado to the Plaza Santa Ana there are several acceptable bar/restaurants including a German-style brewpub called Naturbier. As I've said elsewhere to go tapas and wine bar hopping I suggest Calle Cava Baja nearer to the Plaza Mayor. Few tourists great wines at several bars including Taberna Tempranillo and Galician Orixe which may be a great chance to sample Galician food and wine if you aren't visiting the region. Casa Lucio is on this street as is their more afordable and more accesible sister restaurant.