Pinot Noir (World)
Beaujolais (France)
Dolcetto (Italy)
Pinot Gris (France)
Gewürztraminer (France)
Prosecco (Italy)
Cava (Spain)
Sekt (Germany)
Champagne (France)
Onion Soup is full of sweet, caramelized onions, crusty bread, and cheese. It needs bright fruit (cherry/raspberry/strawberry) to pick up on the onion's sweetness.
Food friendly and fruity wines like Pinot Noir, Beaujolais, or Dolcetto will do the job.
On the white side, a Pinot Gris or a Gewürztraminer will bring out the flavors of the soup.