Whisk together creme fraiche, sherry vinegar, whole grain mustard, and extra-virgin olive oil in a bowl.

Fold in the chopped chives.

Set the creme fraiche mixture aside at room temperature.

Toast the raw hazelnuts in a dry pan over low heat for 4 to 5 minutes, or until golden and fragrant.

Remove the toasted hazelnuts from the heat and roughly chop them.

Set the chopped hazelnuts aside.

Bring a pot of salted water to a boil.

Blanch the haricots verts (trimmed green beans) in the boiling water until bright green and crisp.

Drain the blanched green beans immediately.

Toss the drained green beans with the creme fraiche mixture.

Season the mixture with salt and pepper to taste.

Garnish the green bean casserole with the toasted hazelnuts before serving.