How to Make It
In a food processor, pulse the flour and salt. Add the butter and pulse until the mixture resembles coarse meal with some pea-size pieces of butter still visible. Sprinkle in the ice water and pulse until the dough starts to come together; you should still see small pieces of butter. Scrape the dough out onto a work surface and pat into a disk. Wrap in plastic and refrigerate until chilled, at least 1 hour or up to 3 days.
On a lightly floured work surface, roll out the dough to a 14-inch round. Transfer to a 10-inch springform pan and press in gently; it should reach halfway up the side. Refrigerate for 20 minutes or until firm.
Preheat the oven to 375°. Spread the walnuts in the chilled crust. In a medium saucepan, melt the butter over low heat. Add the brown sugar and corn syrup and stir until dissolved. Add
the vanilla, orange zest and a pinch of salt. Whisk in the eggs. Pour the filling over the walnuts, then fold the dough over the filling in an overlapping pattern. Sprinkle the crust with the granulated sugar. Bake the galette for 40 minutes, until the filling is set and the crust is golden brown. Transfer to a rack and let cool completely.
In a medium saucepan, combine the cream, milk and vanilla bean and seeds and bring to a simmer over moderate heat. Remove from the heat, cover and let steep for 20 minutes. Discard the vanilla bean.
Set a heatproof medium bowl over a bowl of ice water. Return the cream mixture to a simmer and remove from the heat. In another medium bowl, whisk the egg yolks with the sugar until pale; slowly whisk in the hot cream. Pour the mixture into the saucepan and cook the custard sauce over moderate heat for 1 to 2 minutes, stirring constantly and scraping the bottom and side of the saucepan, until thickened. Pour the sauce into the bowl set over ice water and let cool completely, stirring occasionally. Stir in the bourbon and refrigerate.
Unmold the galette and transfer it to a plate. Drizzle with maple syrup and sprinkle with sea salt. Serve with the custard sauce.