For the caramel, combine the sugar, maple syrup, and 1/3 cup water in a small, deep, heavy-bottomed saucepan. Bring to a boil, swirling the pan (don't stir!) to dissolve the sugar. Cook at a low boil without stirring for 5 to 10 minutes, until the mixture turns a golden brown and registers 230 degrees F on a candy thermometer. Watch it carefully so it doesn't burn! Off the heat, swirl in the fleur de sel, and immediately pour into an 8-by-2-inch round cake pan (not a springform!). Set aside to cool for 30 minutes.
Meanwhile, place the sweetened condensed milk, evaporated milk, canned pumpkin, and mascarpone in the bowl of an electric mixer fitted with the whisk attachment and beat on medium-low speed until smooth. Whisk in the eggs, vanilla, maple extract, orange zest, cinnamon, and nutmeg. Gently pour the pumpkin mixture into the pan with the caramel so they don't combine.
Place the pan in a roasting pan large enough to hold the cake pan flat and fill the roasting pan with enough of the hottest tap water to come halfway up the sides of the cake pan. Bake in the center of the oven for 70 to 75 minutes, until the custard is just set. It will be firm but still jiggle slightly in the middle; a knife inserted into the center of the flan will come out clean. Remove the flan from the water bath, place on a cooling rack, and cool completely. Cover with plastic wrap and refrigerate for at least 3 hours. Don't tilt the pan or the caramel will run out!
Run a small knife around the edge of the flan. Turn a flat serving plate with a slight lip over the cake pan and flip them, turning the flan out onto the plate. The caramel should run out over the flan. Cut into wedges and serve with the caramel spooned over each slice.