Our meal at a Casa de Pasto - Estrela do Minho - was the best we had in Lisbon. Located on Rua dos Bacalhoeiros #137 (about a 5 minute walk from Sao Jorge castle) it's non-descript from the outside and very modest on the inside. For under 10 euro you get a juicy piece of cod smothered in a delicious sauce with tomatoes, onions and olives with a side of batatas fritas and a mixed salad. With generous portions, friendly service, and delectable food at a great price you really can't go wrong. Follow it up with a serving of white cream to help you digest. Highly recommended - this place inspired us to write our first review on chow hound!
p.s. across the street you'll find a popular fish cannery that's been in business for over 80 years. The tins alone are worth a visit - with special editions designed by local musicians.