Start by making the crust. Place hazelnuts, flour, sugar and sea salt into a food processor and pulse until the texture resembles a coarse meal. Add butter and pulse a few times until the butter is the size of peas. Add water and pulse just until the dough starts to come together. It will look crumbly, but should hold together when pinched. Add a little more water if needed.
Transfer the dough to an ungreased 4 x 14 tart pan with removable bottom, pressing the dough evenly into the sides and bottom of the pan. Cover with plastic wrap and chill in the fridge for at least 20 minutes (or up to 1 day). Preheat an oven to 350°F and set an oven rack to the middle position. Place the tart pan on a rimmed baking sheet, and bake until the crust is lightly golden (20 - 25 minutes). Transfer to a wire rack and let cool for 15 minutes.
Meanwhile, prepare the pumpkin filling. To a large bowl, add pumpkin puree, brown sugar, white sugar, flour, cinnamon, ginger, nutmeg and salt, stirring to combine. Add the egg, egg yolk, cream, vanilla and bourbon, and stir until smooth. Place the tart pan back on a rimmed baking sheet, then pour the filling into the cooled tart crust just to the top of the crust. Not all of the filling will fit in the crust - you'll have just a smidge leftover.
Bake tart until the filling is just set (30 to 35 minutes). Transfer to a wire rack and let cool completely (about 2 hours).
Just before serving, top the pumpkin tart with an even layer of marshmallow halves. The cut side down and the rounded side up. You should be able to fit 9 or 10 rows of 3. Place the tart under a hot, preheated broiler just until the marshmallows turn golden brown. Keep an eye on this as the marshmallows can burn quickly, and move the pan around as needed to ensure even toasting.