Preheat oven to 325˚F with rack in the middle.

Toast hazelnuts in a small baking pan until centers are golden, 15-20 minutes (cut one open to test for doneness). Cool to warm, then rub off any loose skins in a kitchen towel. Coarsely chop nuts.

Cook beans in a 6 quart pot of well-salted boiling water until just tender, 4 to 6 minutes. Transfer to an ice bath to stop cooking. Drain and pat dry.

Whisk together mustard, vinegar and 1/8 teaspoon of salt in a bowl, then add olive oil and hazelnut oil (if using), whisking as you drizzel the oil. Add beans, nuts and onion and toss well.

Season with salt and pepper and serve.