Our tour guide told us that any restaurant serving special meals for each day is an authentic restaurant! And this restaurant was precisely that. With dishes of the day written on the board outside, to the crowded place and the people running around- this place was everything that was told as an indication!
The first time we tried a Francesinha, the Porto speciality. Since we don't eat beef or pork, we got a custom made chicken Francesinha, which was just delicious and pretty affordable (6.5€). The waiter forgot to bill me for a coffee that I ordered and when I told him about it, they put it on the house! I really loved the food, the place and the friendly service!
Later again we tried this place for dinner. And this time we ordered pepper prawn pasta, which was pretty good too! And its a big enough quantity for 2 people. So it's pretty cheap (10€). Since our dinner was taking too long, he even offered olives on the house! It's not a lot, but it's an incredibly sweet gesture.
I would definitely recommend this place to tourists, and the local people alike.