This cosy basement restaurant has become a regular destination on our trips to Zermatt (5th visit since 2008 just this week). We go in November so not everywhere is open, and stumbled upon this place on our first trip in 2008 when we were looking for somewhere else entirely. The atmosphere is wonderfully cosy and the service has been consistently friendly. The menu is simple, though high quality, mostly steaks and lamb dishes. You can have the usual fondues, but in reality you should go for the meat options. They are beautifully cooked and of very high quality. For those who love their rosti-they were quite happy to change the vegetable offerings. The wine list is HUGE. It's pricey-but that's Zermatt for you. This time we actually stayed in the Hotel Julen which sits on top, so all we had to do was take the lift to the basement floor.
I would guess in midwinter you have to book to get a table, though in November you can just walk in. I cannot imagine having a bad meal here and thoroughly recommend it!