ROAST AND CARAMELIZE THE NUTS
Preheat the oven to 350 degrees F. Line the bottoms of 2 sheet pans with parchment paper.
Spread the hazelnuts on 1 prepared pan. Mix the pumpkin seeds and pistachios together and spread them on the second prepared pan.
Roast the hazelnuts until the skins start to darken and shrivel, about 8 minutes.
Roast the pumpkin seeds and pistachios until lightly toasted, 8 to 10 minutes.
Remove the nuts and seeds from the oven. As soon as the hazelnuts are cool enough to handle, rub them in batches in a kitchen towel to remove the skins. Not every bit will come off.
Roughly chop all of them and mix together in a bowl.
MAKE THE BARK
Line the bottom of an 8-by-12-inch sheet pan with parchment paper.
Pour the tempered chocolate into the prepared pan. Spread it evenly with a small offset spatula. Tap the pan on a work surface to even the top.
Sprinkle the roasted nuts and seeds and the fleur de sel over the chocolate. Tap the pan again to settle the toppings into the chocolate. When the chocolate loses its sheen and starts to set, after about 15 minutes, cut the bark with a sharp knife into 2-inch squares or other fanciful shapes of your choice.
Leave the bark at room temperature until it is completely set, about 1 hour, and then separate the pieces.
Store in an airtight container in a cool, dry place, not in the refrigerator.