How to Make It
Preheat the oven to 375°. Unfold the puff pastry onto a parchment paper–lined baking sheet. Pierce the pastry all over with a fork, brush with the egg and sprinkle with fleur de sel. Top with another sheet of parchment paper and a second baking sheet. Transfer to the oven and set a heavy ovenproof skillet on the top baking sheet. Bake for 30 minutes, until the pastry is lightly golden and almost cooked through. Remove the skillet, the top baking sheet and the parchment paper and bake for another 20 minutes, until the pastry is cooked through, golden and crisp. Transfer to a rack to cool completely.
Meanwhile, spread the hazelnuts in a pie plate and toast in the oven for 6 to 7 minutes, until golden. Let cool slightly, transfer to a clean kitchen towel and rub off the skins. Chop the hazelnuts.
In a medium heatproof bowl, combine the chocolates. In a small saucepan, combine the crème fraîche, rum and sugar. Whisk over moderately low heat until simmering, then pour over the chocolate; whisk until smooth. Pour the chocolate evenly over the cooled puff pastry, smoothing the top. Top with raspberries and sprinkle with the toasted hazelnuts and fleur de sel. Cut into pieces and serve.