Which wine would be a good pairing with french onion tart?

Sancerre

Depends upon your mood, and whether your diners prefer red or white. Or you could ahve one of each. For me that particular tart says Alsatian white... Pinot Blanc preferably, or could be Riesling. For a red, I'd like a village Bourgogne from the Cotes de Beaune area.

I'd serve a Kabinett Mosel for this -- crisp and refreshing, and just a bit off the beaten path! ;o)

A Gewürztraminer from Alsace! Riesling would also be good, as would a Grüner Veltiner.

An Alcatian white, a Pinot Gris, or a light red, like the Beaujolais HalfPint suggests. You want some acidity, which is why the Chardonnay wasn't on HalfPint's list.

Just found this cool wine-food matching site:

http://www.wine-food-matcher.com/

Looks like a sauvignon blanc and any red (Bordeaux, Beaujolais, etc), even a rosé, will pair well with an onion tart. Though I don't see chardonnay on the list for whites.