I've only made a few quick trips to Lisbon so I'm no expert, but I've had a good meals of charcoal grilled sardines and dourade at Casa da India (Rua Loreto 49). They also do a good job with chicken.
I'd love to hear if anyone who knows Lisbon better can recommend anything similar to Casa da India (or is that a particularly good one?)
I ate a really nice lunch of grilled sardines at a cheerful red-checked tablecloth restaurant at I believe 35 Calcada Carmo, whose name I don't know. I think my grllled sardines were 6 euro. The restaurant is NOT the outdoor restaurant in the largo at the top of the car ramp with many umbrellas. It is at the very bottom of the car ramp, next to a hole in the wall that sells cherry liqueur ginginha by the glass.
I was also advised that Zapata, at Rua Poço dos Negros 47, was a nice place to eat traditional Portugese dishes, but when I arrived for lunch, they were closed for a holiday. A shopkeeper next door directed me further down the street to a restaurant "with a yellow door." We walked about two blocks on the same street (the street numbers were going up, not down) until we spotted the "yellow door". I had a hefty serving of grilled mackerel for well under 10 euros. It came with potatoes and greens, which were good, and when the waiter saw me filching my husband's delicious rice (he was having grilled squid), the waiter brought me an extra dish of rice. I never noticed if the restaurant had a name. It might be "The Yellow Door" for all I know.