Porto Restaurant Recommendations?

If you want casual, I recommend Bufete São Domingos, Largo São Domingos 59, at the foot of Rua das Flores.

Here's an excerpt from my trip report last year:

This became our favorite place to eat in Porto for the rest of our
stay. The food was delicious, the portions more than generous, and the
prices significantly cheaper than the recommended eateries just across
the street. That first time Stan had very good grilled salmon; I had
bifana (spicy pork) on a plate, rather than in a sandwich, the more
typical presentation. Both were excellent, and the servings so
generous we couldn’t finish our meals. Cash only, by the way.

A side note: I just looked this place up on the internet to verify the
address, and the most of the reviews are terrible. Go figure.

We found this place while on a Rick Steves tour. Our guides had recommended a couple of places to eat near there, but we chose the less expensive Bufete across the street instead, and were very glad we did.

I think we ate there at least two more times after the tour (we stayed on another 4 days, I think) and were delighted with the food every single time. Stan continued to get the salmon, and each time it was better than before. I tried several different dishes, including a breaded pork cutlet that came with great seasoned rice, and an excellent tuna salad.

I don't know if they take reservations, but it wouldn't hurt to ask. They were usually crowded when we arrived, but one of the staff would tell us about how long it would take before a table would be available. I believe there is some inside seating, but choose the outdoor area, unless the weather is miserable. Great view, good people-watching, and if you're lucky, some street performers as well.

As I mentioned in the excerpt above, it was cash only; no cards accepted. This was in 2023.

Thank you Jane for the recommendation, we will definitely check out Bufete.