With sour cream, mustard, and raw onions,
a semi-dry white wine will work best:
Gravlax is cured salmon, usually accompanied by Beer and Aquavit.
A good wine will also do, but steer away from red wine.
The wine should be extra fresh (acidic) and fruity to cope with both fat, salt, sugar, and umami. Creamy and citrusy Treixadura is a very good choice.
Slightly fizzy Alvarinho from Portugal can also be a good match.
Match Gravlax with pink wine! Rosé wines from Piedmont (Nebbiolo Rosato) often have both fruit and freshness enough to contrast the fat fish.