I ate at a restaurant serving Qinghai specialties in Beijing last fall. I can't really tell you for sure if the Qinghai cuisine is or is not "Tibetan", but as the main minority in Qinghai is of Tibetan ethnicity, I would venture to say that the two cuisines are similar to a large extent and they are culturally close.
I couldn't tell you the names of the dish I had but I clearly remember the beer (Lhasa) for having had a few too many...All I can tell you about the food is that it was delicious and very different from Chinese cuisine. I had a great lamb rack with some pretty tasty mushrooms served with cold noodles, tomatoes and chili peppers, and some steamed eggs in chicken broth and small dumplings that tasted a bit like italian gnocchis. Another dish was thin slices of either beef or yak, with thin strips of vegetables like carrots and a long green asparagus-looking vegetable that I never saw before.
I cannot remember the name of the restaurant, but it is on Zao Ying Street in Chao Yang district, and it is in a class of its own, very classy decor. The service and the quality of the food was as good as in any 5 star restaurant.