In a 6- to 8-inch frying pan over medium heat, stir hazelnuts often until golden under skins, 5 to 10 minutes.
Pour onto a towel and let stand until cool enough to handle. Rub nuts in towel to remove any loose skins. Lift nuts from towel and reserve; discard skins. Coarsely chop nuts.
In a large bowl, whisk vinegar, oil, and mustard to blend.
Cut pears lengthwise into quarters and core; cut quarters lengthwise into thin slices. Drop slices into bowl and mix to coat with dressing.
Add spinach and mix gently.
Sprinkle with cheese and toasted nuts and mix gently.