Having finished a trip through Northern Portugal (between Coimbra and Spain) I´d like to share some experiences. I have to admit that probably I would not go to Portugal for the food. There is excellent fish, seafood and beef but nearly everything was just nicely grilled with some olive-oil added, good but rather bland. I had expected a former colonial power to use spices but they don´t, nor herbs, not even pepper (salt could be added on the table, and was, even by those of us who use it sparingly at home).
The better places we encountered:
Across from Porto, on the southern side of the river Douro, there is a riverwalk with many glasshouse-style feeding troughs. We had dinner in the first glass-container, that is the one nearest to the bridge and we all (4) considered it one of the nicest tourist-traps we ever fell into. It is of course touristy but so were we, the crew was charming, the food good and the sight onto the Porto city-scape (especially when it got darker) splendid. They bring different appetizers to the table and you just order which ones are going to stay but we were content that we did not send back anything. We had different kinds of fish for main course and all were nice.
In Viseu we had dinner on 2 consecutive days in "Muralha da Sé", across from the cathedral and to the left of Misericordia church. Nice to sit outside but indoors as well. Everything we had was good, try the black pudding sausages with pineapple cube in a Port-cinemon sauce as an appetizer (but share it or you will struggle with the main course); both fish and beef main courses were excellent.
In Viana do Castello we had a really rustic dinner in a small restaurant in the main pedestrian street (the first of three restaurants in a row, but the only one which was full the whole evening), again excellent fish and a very efficient crew, we were running up a bill of just 42 Euros for 4.
We tried to stay in Pousadas most of the time, the food there nice but not exceptional and rather pricy compared to other restaurants, but wuth splendid and stylish surroundings.