For the dressing:
Whisk all ingredients except hazelnut oil together. While whisking, slowly drizzle in hazelnut oil. Season with salt and pepper, to taste. Set aside until ready to serve salad.
For the salad:
When ready to serve toss greens, asparagus and vinaigrette together, seasoning with salt and pepper. Place a generous handful on each salad plate, top greens with onions and sprinkle with hazelnuts.