Toast the hazelnuts on a baking sheet until lightly golden brown, shaking the pan occasionally to jostle the nuts and ensure they cook evenly, about 10 minutes. Cool completely. Rub the hazelnuts between your palms to remove the dark skins from the nuts. Very coarsely chop the hazelnuts.
In a blender, blend the shallot, mustard, garlic, and 2 tablespoons of the vinegar until very finely chopped. Add the remaining 2 tablespoons of vinegar and blend until smooth. With the blender on low, slowly drizzle in the oil until emulsified. Season the vinaigrette to taste with salt and pepper.
Mound the salad on 4 plates, dividing equally. Sprinkle with the hazelnuts and serve.