1 cup each whole roasted salted cashews, whole large pecan halves, whole unsalted almonds, and whole walnut halves (4 cups total)
1 1/2 cups sugar
2 teaspoons pure vanilla extract
2 teaspoons kosher salt
1 teaspoon fleur de sel
Preheat the oven to 350 degrees.
Combine the nuts on a sheet pan, spread them out, and roast them for 7 minutes, until they become fragrant. Set aside to cool.
After the nuts are cooled, place the sugar and 1/4 cup of water in a medium (10-inch) saute pan and mix with a fork until all of the sugar is moistened. Cook over medium-high heat until the sugar melts -- from this point on, don't stir the caramel, swirl the pan! Don't worry -- the mixture may look as though it's crystallizing. Continue to cook for 5 to 10 minutes, until the mixture becomes a clear golden brown, swirling the pan constantly at the end. (Careful -- the caramel is very hot!) Off the heat, quickly add the vanilla (it will bubble up!) and swirl the pan to combine. Working quickly (the caramel will continue to cook in the pan), add the nuts and the kosher salt and toss with 2 large spoons until the nuts are completely coated.
Pour the nuts and any extra caramel in the pan onto a sheet pan lined with parchment paper. Spread the nuts out in one layer, pulling them apart with two forks. Sprinkle with the fleur de sel and set aside to cool. When they're completely cooled, carefully break the nuts into large clusters with your hands, trying not to break the nuts too much.