I enjoy a good quality Alsatian Pinot Gris such as Albert Boxler or a rich Oregon Pinot Gris. The wine flavours need to be rich to match to the onions and the acidity racy to counter the butter and creaminess in the dish.

http://rouxbe.com/recipes/100/preview

It is amazing... Can't wait to try the pairing... Cheers,

While I agree whole-hearted with John's recommendation, I really like an Alsatian Riesling (Dry) with this type of tart.
Another thing that I commonly do, is to top the tart with sautéed fresh Morels and a sprinkle of freshly grated Reggiano before it goes in the oven. Then I move on to a Pinot Noir (Burgundy/Beaune) for this pairing.

What is the difference between Pinot Gris and Pinot Grigio. Is it simply that one is from Italy and the other from the U.S.? And if so why are they called two different things?

Either way you say it, I do like my Pinot Grigio. That being said, I have also tried some Alsatian Riesling, that I thought were really nice.

Pinot Gris / Grigio are the same thing. Actually, in Alsace, they sometimes call it Tokay Pinot Gris (not to be confused with the Hungarian Dessert wine, Tokay). These are regional differences in the name only. Grigio seems to be what they call it in Italy where the best examples come from the Alto Adige/Trentino region. If you're a big PG fan, you have to try the ones made by Elena Walch as her single vineyard versions are some of the best in the world. The 2006 Castel Ringberg is amazing. Another excellent choice is any of the Alsatian versions made by Zind-Humbrecht. Truly artisan wine but a distinctively different in style than the Italian and new world versions.
To top that off, there are also some pretty good examples available from the Okanogan and New Zealand. Cheers!