I went to a restaurant in Cairo once where there was only one dish, called Koshari.
The only choice I received was small, medium, or large.
I went for medium, which turned out to be a good idea as the portion was huge.
This is an excellent example of when keeping things simple and not giving customers a choice really works out.
The dish was a mix of rice, noodles, pasta, lentils, chickpeas, fried garlic, tomato sauce, and an optional spicy sauce that you could throw in.
The restaurant was quite full, and my food came right away as of course they are constantly serving up this dish.
While serving one dish only is a clever twist on the restaurant idea, it does put pressure on ensuring that the dish is excellent, and the service is fast and friendly because there is nothing else that matters.
Overall, a brilliant idea, executed well.