Bring a large pot of well-salted water to a boil. Mix onions and vinegar in a small bowl; sprinkle with a pinch of salt.
Boil beans 3 minutes for crunchy beans or 5 minutes if you want them more tender. Drain; plunge into a bowl of ice water.
Drain vinegar from onions into another small bowl and whisk in mustard, 1/3 cup oil, and salt and pepper to taste.
Toss beans with 2/3 of onions, 2/3 of hazelnuts, and vinaigrette to taste. Sprinkle with remaining hazelnuts and onions.
Note: Nutritional analysis is per serving.