My wife is a Lisbon native and I know the city pretty well. At first glance, O Painel seems like any of the small traditional eateries that are all over town.
But although you'll find many of the same dishes at other places, here you'll find that they are cooked with greater care using the best ingredients. You'll also notice that despite the place looking a little shabby, the clientele looks well-heeled. This is a favorite place among Portuguese businessmen who will come from all over town to eat here.
Unless you have a strong preference otherwise, I recommend you choose the daily specials. We were there yesterday and had a great Cabrito (Goat stew) and some Pataniscas de Bacalhau (Cod fish cakes). They come in full and half orders. A half order is enough for one person.
They have a good selection of wines as well. Please don't order Sangria. Save that for the touristy places.
If you own or manage O Painel de Alcantara, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.