That's an easy one for me - it was at Tromba Rija in Villa Nova de Gaia this past summer.
Last spring I attended a wine tasting at a local store here in Seattle hosted by Fernando, the master blender for Kopke/Rocha/Barros. After the tasting I told him that my wife and I would be visiting Porto that summer and asked him to recommend a couple of good restaurants. Among others, he suggested Tromba Rija.
While roaming around in VNdG, we just happened to stumble upon Tromba Rija while walking along the waterfront so we stopped in and asked if we needed reservations for the evening. The manager said it wouldn't hurt, so we made them for 8:00 pm.
Being typical American tourists, we showed up right at 8:00 pm. The restaurant was closed, with no one in sight. We stood there confused for a couple of minutes, then started to head back to the hotel. The manager must have seen us, because she came running out to unlock. Silly Americans, 8:00 pm is an early dinner in Portugal! They were just preparing to open. She told us to sit wherever we liked and said she'd be right back to take our drink order and explain the process.
She didn't even ask whether or not it was our first time - were we that obvious?

- and launched right into an explanation of the restaurant. Tromba Rija is a buffet... sort of. They have a table of cheeses, a very large table of various appetizers, side dishes, and prepared entrees, another table of meats and sausages, and a table of desserts. She explained that the chefs prepare whatever they feel like, so no two nights are exactly alike. In fact, no two HOURS are exactly alike as the chefs replace dishes that have been consumed. She then asked what we'd like to drink, which went something like this:
Manager: "So what would you like to drink?"
Kari: "Do you make a Port Tonic?"
Manager: "No, what is that?"
Kari: "Hmm... okay, could I get a glass of Rose?"
Manager: "No, we don't have any Rose."
Kari: "Oh, okay. What kinds of red do you have?"
Manager: "Just our house wine, it's very good."
Kari: "I'll have a glass of the red, then."
Manager: "I'm so sorry, we don't serve by the glass."
Kari looked at me and laughed that at least she only had to stumble back to the hotel after dinner, but that I had to promise to make sure she didn't fall off the bridge into the Douro! I assured her that I wouldn't let that happen, so she ordered a bottle of the red. (I don't drink wine, just Port.)
The main table of food was fabulous - there must have been 30-40 plates of different dishes. Admittedly, my wife and I can be picky eaters, so a lot of the dishes weren't appealing to us visually. (They all smelled fabulous!) But we managed to have 2 big plates each along with some cheeses and meats, and then shared a big plate of desserts. We were stuffed, and the food had been spectacular!
Coincidentally, about the time we were finishing up our desserts the people at the tables next to ours started smoking. I'm allergic to cigarette smoke, so needed to leave. When one of the waiters appeared to take our latest round of plates, I made use of my limited Portugese and asked him for the check. He got this stricken look on his face and started talking in Portugese far more rapidly than I could ever hope to follow, so I told him I only spoke very little Portugese. I think he told us not to run off, and he rushed off to find the Manager.
She came running out in a panic, too, and frantically asked what was wrong. I was flustered and told her that nothing wrong, the evening had been very pleasurable and the food was fantastic. We were very happy, but it was getting late (it was probably 9:45 by then) and we needed to get back to the hotel and get to sleep. She responded something to the effect of "but you haven't eaten very much!" I explained that we'd had 2 full plates of food each, had sampled the excellent array of cheeses and meats, and had also very much enjoyed the chocolate mousse and other desserts. "But you're not done yet!" was her response.
So then I played what I figured would be the trump card - I explained that I was allergic to cigarette smoke, and since the parties at the adjoining tables were smoking I needed to leave. She seemed relieved, but insisted that we move to a different table in the corner by the door where it was better ventilated (and far from the people smoking). We figured that we were having some sort of language barrier problem (though her English was nearly perfect) so went ahead and changed tables.
"Better?" she asked, to which I replied that yes it was much better. "Ok, good, I'll get your next course." And off she went.
... huh? Next course? MORE food??? ...
She returned with a huge plate of fruit... grapes, banannas, kiwis, strawberries, pineapple, a couple kinds of apples, and a couple of things I didn't recognize. HUGE. This was a Costco veggie platter-sized dish. No possible way that two people were going to eat all of it. We surreptitiously observed others in the restaurant and figured out that we were free to sample what we wanted, so we did.
After a bit we were able to catch her eye and indicate that we were finished, so she brightly removed the fruit.
And returned with a giant bowl of nuts. Almonds, walnuts, pecans, hazelnuts... and probably more. Before we could say anything she zipped off saying she'd be right back. She returned immediately with 3 bottles, some glasses, and a lemon. One of the bottles was obviously Port, and one other was clearly frozen and frosting over.
She explained that the frozen bottle was some sort of almond liquor, that the decanter was their house Tawny Port, and that the 3rd bottle was (waving her hand dismissively) "fire water." She then proceeded to pour us each a shot of the frozen almond liquor, cut the lemon in half and squeeze fresh lemon juice into each shot glass, then stand there expectantly. So we toasted the evening and drank up! It was really very good, and she was very happy to pour us another one each. To this day I don't know what the almond liquor was... she didn't know the right words in English and I didn't recognize the Portugese. She poured me a glass of their house Tawny and left us alone again. So I had some nuts and Port, and then finally got up enough courage to try the "fire water" which she'd left there at the table.
All I can say is... it is aptly named. I asked her later and found out that it was aguardente, the stuff they add to Port to arrest the fermentation. That was fun. I'd intended to try some while in Portugal, so that worked out perfectly.
The waiter eventually came around and took the nuts and alcohol. After another couple of minutes the manager returned again... with more food! Well okay, it was just an after dinner sherbet, but by that point we were feeling like the guy in Monty Python... "it's just one thin mint!" It was also nearly 11:00 pm and we were really getting tired.
But the best part of all... the entire bill for the evening, everything included, was 59 Euros. It is apparently a fixed price all-you-can-eat menu for 29.50 Euros per person, and that includes the bottle of wine! And it really was fabulous, we just wish we'd known ahead of time so that we could have better arranged our days (so as not to be so tired).
I
think that Portugal has now banned smoking indoors like most of the rest of Europe, so we'll be taking my parents to Tromba Rija this fall.