Our concierge recommended this restaurant, and he earned a gold star for doing so. It's tucked away behind the ducal palace, and is a bit hard to find (it helps to remember the street number -- 32 rue de l'Eau), but well worth the hunt. It's located in a 500-year old building, and the main dining room has an ambience to match, without any stuffiness or pretension. Very friendly and efficient staff, excellent food. It was a high point of our visit to Luxembourg.