If you need to roast and peel the hazelnuts* start with that below.

Add a splash of olive oil to the skillet over medium heat. Slice the onion into long strips and add to the skillet. Saute and toss occasionally until the onions are wilted and starting to become golden.

Thinly slice the apple and add to the onions. Whisk together the mustard, vinegar, salt, pepper and the rest of the olive oil. Pour into the skillet and turn off the heat. Toss.

Place the washed and dried spinach in a large bowl. Add the hot ingredients, hazelnuts and toss to combine. The spinach will wilt and sigh. Serve while still warm.

To roast and peel *hazelnuts:
Preheat the oven to 375. Spread on a baking sheet and heat for 15-20 minutes. Shake the pan occasionally. When your home smells like hazelnuts, they are probably done. The nuts will appear golden.

Set the baking sheet aside for 30 minutes. This will cause the nuts to shrink and the peels will start to wilt away.

Next, place the cooled nuts in a dish towel. Roll around in the towel to rub off the skins. Some will stay on. This is fine, removing the skins is more of an aesthetic purpose, although some consider the skins to be too bitter.