Starters

You want to try the "tapas" (petiscos). These are not Spanish tapas, mind you. Portions slightly bigger, consider 2/3 per person. I liked the smoked (actually fired) sausage and the polvo verde.

Main course

Bacalhau (codfish) is EVERYWHERE. Cooked in every possible way or sauce. In general, any kind of fish in Portugal is very good (salmon, clams, prawns).

In addition to fish, a traditional Portuguese dish is Francesinha, a sandwich with, like, 5 different kinds of meat, cheese, a fried egg on top, drowned in a beer sauce and surrounded by fries. If this won't give you a heart attack, I don't know what will.

As a wise Portuguese said,

Everybody can make a sandwich;
the sauce is what makes the difference.

So I went to THE sauce place, where the sauce is supposed to be spicy as hell (they call it "the men sauce". Sexist, uh?).
TL;DR: wasn't spicy; didn't love it.

Dessert

A famous (and cheap) pastry is pasteis de nata, made with custard cream. If they ask you for more than 1 euro per piece, you're in a tourist trap! Don't buy it.

Finish your meal with a good Port wine (I personally like ruby, but tawny is always a good choice) and an espresso.

(and since you're in Portugal, add a wine tasting tour. For good measure.)