The online recommendations all say that Matosinhos is the place to be for seafood. After managing to sneak in to the market just before their unexpected early closing time (Covid times) and purchasing fresh bunches of percebes for €14/kg, we made our way to Meia-Nau (TA #1, Google 4.8) without a booking. The outside tables were all reserved, but inside had space and it was pleasant enough being there out of the heat of the sun.
There is a small counter of the fresh fish on offer and they'll take you through them and weigh before any choices of cooking method. I chose the unsual looking St. Peter fish, which I vaguely recall eating forgettably in Israel (https://eatlikeushi.posthaven.com/unnamed-restaurant-galilee-05-2009).
3€ cover was for the table (not per person) for soft bread, hard rye, tuna and black olives. Fish soup €4.5 was mild but strong vegetable (carrot and bit of tomato) flavour.
Our St. Peter fish was an odd looking character weighing 1.29kg for €58 (€45/kg). It was offered to us grilled or alternatively fried with roe bread soup. They said the fried version was a traditional dish, but we were in the mood for grilled rather than fried food and so went with the more simple option. It had been perfectly grilled to a very salty crisp skin (assumably the excess salt used to dry out the skin), and a soft delicious flesh with mild flavour. The saltiness was balanced nicely with buttered potatoes, cabbage and carrot. It was too much for 2 people, and they packed away the leftovers for us.
It's definitely not a cheap meal (particularly by Portugal standards) but you'll pay a lot more I think for this quality in specialty seafood restaurants in most parts of the world. I would try somewhere different next time (just because they are so many options in the area) but would happily eat here again too.